It's been fine up until now, but it's not like Rockruff has received the lion's share of the limelight so far. If Dusk Lycanroc is going to get more screentime in the future, that means less screentime for Rowlet, Litten, and any other capture he will get. It means there's no way in hell Ash will have both Decidueye and Incineroar without one or both of their evolutions being rushed. And I don't really fancy the idea of an anime-only rival with one of them, as that would basically be Cameron all over again: a shoehorned rival that exists just to prevent Ash from getting Decidueye/Incineroar. That would just be a dick move.
I have no issue with Lycanroc being the ace, although I would orefer something more akin to Hoenn where there are no aces and everyone gets roughly equal screentime. My issue is that they're potentially establishing it as the ace too early, and my concern is that it's going to be another Infernape or Greninja hogging the limelight for 100 episodes and leaving the rest of Ash's Pokemon in the dust. Yeah, it's been fine until now, but there's no guarantee it will stay that way. Hell, Litten is already starved for screentime and battles as it is.
I think it's a little to early to just assume that Rowlet and Litten or any other pokemon that Ash catches is going to get shafted considering that if you look at it, Lycanroc evolved in episode 37 however since then the only real battles it's been where against Team Rocket where it was mainly just practicing a Z-move or was just to end the battle quickly, or in the recent case where it had it's first actual battle being against Gladion's Type:Null.
Now here the thing though it's being said that oh well that Lycanroc battle Gladion's Type:Null so that means it's probably going to be Ash's ace, but here's the thing realistically out of Ash's entire current Alolan Team the only pokemon besides Pikachu that could have realistically taken on Type:Null and win or at least give it a challenge was Lycanroc, and before people try to say that oh well Litten and Rowlet are strong so it's realistic for them to give Type:Null a challenge.
The problem with that argument is that Type:Null was specifically designed to battle Ultra Beast which are just as strong as legendary pokemon, so no matter how you look at it if Type:Null were to either lose or have trouble against Ash's Rowlet or Litten it would make Type:Null look pretty weak and would bring up the question of if it can't beat a Rowlet or Litten, then how could it ever beat a Ultra Beast, and then people would just be bashing it. Also Type:Null beating Ash's Rowlet or Litten wouldn't be that impressive considering you would expect it to be able to easily beat them, so if there trying to show off Type:Null's strength and makes more sense for it to battle Lycanroc or Pikachu.
Yes Lycanroc could still get some more focus and battle Type:Null again but for all we know by the time that Ash and Gladion have their final battle it's quite possible that Rowlet and/or Litten have fully evolved by that point and even surpassed Lycanroc in strength, because you have to keep in mind the reason why Ash's Lycanroc is his 2nd strongest pokemon on his Alolan Team after Pikachu was simply do to the fact that it was the first one to evolve, while Litten and Rowlet are still in their base stages.
Since when does one of Ash’s Pokemon NEED to have its own Z-Move and/or evolve to have a purpose? Litten could never use a Z-Move again and never evolve and it would still have a role as a character and team member. Sure it would kinda suck if it wasn’t getting similar attention to the others, but it wouldn’t make it irrelevant.
I’m not upset about it, I just think a lot of people are jumping the gun. I personally think it would be more interesting to keep him in that role than to go for an obvious choice like a fully-evolved starter, but there is a lot of time left in the saga and things can change quickly. Also, people keep treating the rivalry with Gladion like it’s the defining plot of the series, ala Paul or Alain, and we just have no idea if that will be true. He certainly hasn’t yet developed the personal rivalry with Ash that they did. Regardless, it’s fully possible for Lycanroc to take center stage in rivaling Gladion without being the main focus outside that context. Again, I see nothing wrong with Lycanroc being the team’s ace, and I think it can be handled in an interesting way, but I simply think it’s too early to definitively judge.
Also, on a somewhat unrelated note, I think people are hyping up Silvally’s power quite a bit more than may be necessary. Yes, it is a Legendary Pokemon ... a distinction given to it by the people who MADE IT, who intended it to be a Pokemon mirroring Arceus and being able to “slay” the Ultra Beasts and FAILED. Obviously Silvally is not Arceus, and it is certainly a powerful Pokemon in its own right, but honestly we have no indication that the Aether Foundation even succeeded in creating it as the “Beast Killer” it was meant to be. So yeah, I have no doubts that Silvally is an incredibly powerful Pokemon who will give Ash a good run for his money in their future battles and who will probably play a role in combatting the Ultra Beasts, but I think people are assigning it a lot more grandeur than it really possesses. This isn’t a Darkrai or Latios or whatever that earned its Legendary title by striking awe in the hearts of humans who witnessed its power; it was given its title by a certain green-cuffed Aether dipwad who spends every waking moment stroking his own ego and would just love to claim that he created a wonderful new Legendary Pokemon in the vein of Arceus itself.
Actually from the way things are going if what you just said happens to Litten, then what that would basically mean is that Litten would get shafted, and before you say that it doesn't keep in mind that looking the Alolan Team right now it breaks down like this:
Pikachu has the Electrium-Z and going to be getting the Pikashunium-Z
Rowlet has the Grassium-Z
Lycanroc has the Rockium-Z
basically that's already half of Ash's team that have their own Z-crystal and in Lycanroc's and Rowlet's(if it fully evolves into a Decidueye) case they could even get their own signature Z-crystals. And given that we know that Ash is getting other Z-crystals it's quite likely that Ash's 4th, and 5th(and if they decided to have him catch 3 more pokemon instead of to then his 6th) captures will likely have Z-crystals that match one of their types, and likely if they have evolutions they could easily evolve. So basically at the end of the day what your basically saying is that Ash's Litten should be the only pokemon on his team that doesn't evolve and a Z-crystal that matches it's type basically in all likely hood making it the weakest linked in the team and shafting it.
So doesn't Litten have to have a Z-crystal that matches it's type? No, but given how each of Ash's pokemon on his Alolan team are getting Z-crystals that are matching their types there is really no reason as to think why Litten wouldn't get the Firium-Z, I mean in all honestly if they didn't want to give Ash a Firium-Z then they could have just had Kiawe get Litten to, and no matter how you look at it I don't think it makes any sense for Litten to be the only pokemon on Ash's entire Alolan team not to have a Z-crystal that matches it's type and then on top of that not allowing it to evolve.