Not every movie needs to be serious.
Also, I loved the fact that the original Star Trek had some just plain funny episodes. My favorite episode was very serious ("The Doomsday Machine") but I was happy to watch the funny ones as well ("I, Mudd" and "The Trouble With Tribbles"). It was something I missed with Star Trek: The Next Generation... that show was so serious all the time.
"The Trouble With Tribbles" was a masterpiece, in that it was primarily a comedy episode but there was an actual serious plot underneath, and both worked together. And IMHO the Ant Man movie worked the same way: it was almost pure comedy, but there was a serious plot. There was a big twist that had me actually concerned (oh no, it really would be bad if those particular guys got that particular bit of technology) but the comedy kept on coming.
I'm not sure I will ever bother to watch Ant Man a second time, but I regard that as two hours enjoyably spent and I do recommend it. If you watch the trailers and it looks like fun, you will like it.