The idiom uses the expression as hell in the sense of
(slang) To the maximum degree.
- When I read that passive-aggressive note from my neighbor, I was mad as hell. Oh man, I haven't slept in days—I'm tired as hell.
(The Free Dictionary)
used for general emphasis:
- I’m serious as hell. I’m leaving him. (as) sure as hell: I know what you want, as sure as hell.
(Macmillan Dictionary)
- So, funny as hell means very, very funny.