They don't have to be paired relatively to look more competent than they deserve. They already had Shaymin, who was scared stiff of battling Team Rocket, suddenly fighting with zero fear in the battle royal with no development whatsoever building up to that, jobber opponent or not, something was blatantly skipped there. All they have to do is make Mallow use a margin of tactics and they've already exaggerated things because she's barely learned how to even use basic commands.
The 'well they're not about battling anyway' excuse is moot when other characters' development IS, Ash and Kiawe EARNED their versatility as battlers, learning tactics and experience step by step, there was trial and error. It's the same as suddenly having Ash be a good cook (just tactically not directly shown as good a cook as Mallow so it's not TOO obvious) with no development since his rainbow coloured barf of a failure at the start of the series. It undermines an element that another character had to TRY for when another just suddenly gains a large chunk of that competence out of nowhere. It's even more annoying since battling was largely treated as the one thing Ash had while the girls aced everything else (I feel like a handful of previous episodes let Ash be good at something else for once just so this didn't cause too bad an imbalance).
Except being portrayed as having the same experience as Ash or Kiawe, just because they may do good in some battles doesn't mean their equal to Ash, Kiawe, Guzma, or Gladion in skill level. Your making some pretty big claims here and leaps in logic here, when there is nothing to suggest that they are the same skill level Ash, Kiawe, Guzma, or Galdion. The two having a good battle against each other doesn't mean that they are being more competent then they deserve.
And yes they have trained sure a lot of it may have been off-screen but at the end of the day no matter how much you dislike off-screen training and say that they should show not tell, the off-screen training still counts as training.
Also what does Ash, and Kiawe have to do with anything, the fact of the matter is that neither of those two are battling Mallow or Lana right now so it's not relevant how good they are because aren't the ones battling against Lana and Mallow, and regardless of how the battle between Mallow and Lana goes down does not mean they are some automatically equal in skill with Kiawe or Ash.
I mean to use your thing about Ash suddenly being a good as good cook as Mallow, right now what your trying to do would be like saying that just because suddenly Ash makes a meal that he and the other classmates like that it somehow means he's that he is as good as a cook as Mallow even though we don't actually get a solid comparison of how those two's cooking really stacks up against each other, as just because someone can make a meal that a couple people like doesn't mean there suddenly on the same level as a master chef or someone with more cooking experience, like wise when it comes to battling.