To be fair, look at Wolf's model.
It looks unfinished. Looks good in some areas, but you can tell that it still needs some work done. Especially around the jaw.
Other characters in early stages for the game could've been planned and treated as if they were playable, but then dropped due to various reasons. Time constraints, being too low of priority, etc. If they were dropped as playable during development and delegated into Assist Trophies, it makes sense that their models wouldn't be up to par with most of the current cast. I say most because, again, look at Wolf.
Think of it as a literal progress bar. Characters being developed at different priorities progress through their progress bar faster. If some characters had their progress bar paused/stopped, then their models would make sense.