Been in that spot before, it's a MAJOR risk no matter what the client says. Did a job with a famous mascot once and almost got SUED, learned my lesson!
Clients always swear they have permissions sorted, but half the time they're just guessing. That mascot mess sounds like a nightmare waiting to happen. You really need everything in writing, even for stuff that seems obvious. Got burned once on a local sports team logo and now my contracts are ten pages long.
Seriously, that mascot situation sounds way too familiar. I had a client beg me to use a well known cartoon character in their promo stuff, swore up and down they had the rights. Ngl, I almost did it because the money was good. Thank god my lawyer friend saw the draft and freaked out. Now I make them show me the actual permission paper before I even start sketching. Never again.