Deadpool tosses himself from building. A passerby sees him and comments:
Passerby: I can't believe he's still alive.
What does it mean by "left the house in that shirt"?
It means that Deadpool finds the shirt so ugly or unfashionable or unflattering that he can't believe that someone would actually leave their home and be seen in public wearing it.
It's his brand of humor combined with him trivializing his fall. First, he's dismissing the fall as anything worthy of comment. Second, he's displaying his attitude - insulting pretty much anyone, especially anyone he doesn't like.