Wall*e was... well, no underrated, not overrated... it was in the middle where it was supposed to be. It did well at the box office, it spawned merchandise, people can remember it by name. It did too well to be considered underrated.
As far as Pixar films go, I'd say Ratatoullie was probably the most underrated of the bunch, only because it was pretty much an artistic film with a LOT of gourmand/food snob references that would fly over kids' heads. Plus, I know 2 people who refuse to see it because of their fear of rats, and something tells me they'd HATE the ending scenes where all the rats are running around the kitchen... the merchandise didn't do too well and you never really hear of it anymore. Also, Bug's Life seems to be in the shadow of their other films... it was successful when it came out, the merchandise did well... but it seems to get the cold shoulder for some reason.