I received some great photos of people (grown-ups) dressed up as Babymouse for Halloween. The first is from librarian Lori Ess:
So cute! I love the knee socks.
Meanwhile, the folks at Random House hold a Halloween costume competition each year between the Editorial, Marketing, and Sales departments. This year, members of the Editorial department each went as different Babymouse covers!
UPDATE: Found a close-up shot of super-rad Random Houser Jim Thomas as Babymouse: Rock Star on the PW Halloween costume round-up page, which has many more awesome kidlit costumes: