I think part of the reason that they're looking at Rudd for the role is that Ant-Man is at best one of the MCU's least interesting characters, and at worst one of its least likable. The impression I've got from the cursory reading that I've done is that he's a psychologically fragile and at times selfish scientist. He's also earned a reputation as a possible spousal abuser since the bronze age of 616 and a confirmed spousal abuser in the Ultimate universe. Hell, I think in Marvel Zombies he consciously cannibalizes her.
With respect to the "Is he supposed to be funny?" thing, all the Marvel movies have an element of humor. If you aren't familiar with Edgar Wright, he's the guy who directed the Simon Pegg/Nick Frost Cornetto trilogy (Shaun of the Dead, Hot Fuzz, The World's End) and Scott Pilgrim, so his strength is comedic action. In this case especially, I think they might play the humor up a little more to make up for the fact that the lead is a guy whose powers are to shrink/grow and communicate with insects.
Rudd is a likable actor with comedic chops. He's masculine enough to fill out the suit and "beta" enough to be believable as a fragile/self-conscious scientist type. I'd actually like to see him in the role if for no other reason than that I think Edgar Wright could make a really fun movie with him.