In Hoenn, the mistake was that the one "star" of Brock's team was Lotad. While Mudkip and Foretress were regulated as background characters. Lotad basically "stole" the showcasing that could have went to Mudkip. It wasn't until Brock left Ludicolo at home that Mudkip (and to a lesser extent, Foretress) got some decent screentime. At least, until Bonsly showed up.
Heck, I don't care, since lotad was one of my favourite Hoenn pokemon. Though I do agree the mudkip line got it bad.
I almost have a feeling it wasn't Brock's owning it alone which got mudkip the shaft. Looking at the three evolved pokemon, I notice that sceptile and blaziken are lean, mean, fighting machines, while swampert is a little chubby. Swampert's being ignored might well have to do with its looking more like some sort of water monster than a "cool" fighter type creature, as sceptile and blaziken were. Not only that, but swampert got a quad weakness, meaning it could easily be thwarted by anyone wielding a grass pokemon.
Not to say Brock's owning it, and lotad's foreshadowing it had nothing to do with its being ignored.
But I still think Brock might have done better if he were given a chance with a less shaftable starter. I mean, turtwig doesn't seem the type to be ignored. Maybe being Brock's pokemon might change what people think about it, but then again, mudkip
is the last pokemon on the starter list. Turtwig's first. And Brock's owning turtwig might give it a better chance to fully evolve, as it could replace onix. Ash likes little, nimble critters, which torterra certainly isn't.
Though I guess lording it over smaller pokemon, all the time being doomed to go the way of raichu and be felled by a little critter is better than being all big and intimidating only to never be seen fighting. Still, torterra's fate is incredibly, painfully predictable.
Chimchar would be wasted potential on Brock, and piplup would just be bizarre.