Since the mascot creature generally accompanies one of the main characters, it shows up a lot, and they can almost always speak, too (sentient). For those reasons, I feel like the mascot is basically getting as much attention as a main character would, and should essentially
be a main character, as in, it needs to have a developed personality and serve as much of a purpose as any other main character does (at least for the sentient mascots). When done like this, I quite like them.
But, I feel like using using such characters as nothing more than cute pets (as in, they don't do anything else but look cute) is cheap--if that's all it's going to do, don't make it sentient, and don't give it so much attention.
________________
Manga Cover Database