It might be heretical for an atheist to “believe in” anything, but I don’t know if I’d call it hypocritical. On the other hand, it’s human nature to be “hopeful” of things or to “hope against” some other things.
For example, I don’t “believe in” evolution, but I believe that the theory of evolution is a good explanation for the development of life on this planet into differentiated species. To “believe in” sounds faith-like to me, and I don’t have a lot of faith, even in very good ideas. Some just seem more worthy of “believing” (not “believing in”). Maybe that’s too subtle.
I don’t “believe in” luck, either, but as an impartial and intelligent observer of others from time to time, I understand how some people can often appear to be luckier than others. Many times that’s a product of hard work, preparation, intelligent planning and deployment of resources, game-playing or whatever that makes a consistent winner appear to be “luckier” than his competitors. (I get “lucky” a lot playing cards, especially when I play team games with another intelligent partner who understands the game as well – or better than – I do, can anticipate why I play in a certain way and either support or override my strategy with a better one… and we get “lucky”.
Then, of course, unless you’re a fatalist or believe in total predestination, you can see as well as anyone that sometimes people do get lucky – or the opposite, luck gets them.