I think, because what's 'humourous' or plain old 'funny' varies so widely, the only people who end up in the 'humor' category are people with a name for being funny: established comedians, or commentators, etc. People that you get a sense of their writing from their writing personas. If you like 'folksy' you might be appalled at 'dark satire'.

If you don't fit that category (already well-known humorist) your best bet is to pick a genre and tout your book as a humourous example of that.