My take on it would have to be that each case is unique. I mean in the Secret of the Ooze we saw Tokka and Rahzor as drooling morons. Sure they were "babies" which doesn't really make sense to me why Tatsu would grab babies but whatever. And you see Tokka and Rahzor in Turtles Forever, and obviously they don't have any lines or anything to disprove that they are any less stupid in TF as they were in Secret of the Ooze. So I guess its kind of fifty/fifty or up to chance. I think Baxter was definitely dumber as the fly than he was as a human. With Bebop & Rocksteady its tough to say as they seemed pretty dumb as humans, yet they were definitely dumber season 2+ due to the writing but one could argue that they were in fact dumber post mutation. And for the dog guy, maybe he was just all amped up for battle so his angression mixed with animal mutation and traits made him into a drooling hyped up adrenaline infused animal.
As for the fly, yeah he shoulda mutated into a human, maybe on the animal side its not an instantaneous thing, and the human traits woulda came later? I really enjoy Turtles Forever so I'm trying hard to make it all work
