Miguel, in order to look more like a skeleton, tries to walk in a shaky manner, only for Hector to point out that skeletons don’t walk like that, and to that the kid replies “You walk like that” (naturally Hector tries to deny that).
Both are right: with so many skeletons around, only Hector walks in such a way; indeed, he doesn’t even wear shoes, and his feet don’t look all that right.
His heel bones look bigger than normal, and I bet that’s why he walks in such an awkward manner.
I’m glad that, by the end of the movie, they got him a pair of shoes.