Yes. It's called Adventure Time: Hey Ice King why'd you steal our garbage?!!. It's in the E-Shop at a slightly lower price than a regular 3DS game. Hmm. Check the Adventure Time video there if you want also.
Yes, there IS an Adventure Time game, and it's for both the 3DS and DS. You can find it at places like Gamestop, Best Buy, Amazon, or the Nintendo eShop.
My friend bought it and is supposed to let me borrow it in a couple weeks when I see him. I enjoyed the demo decently, but I agree the combat system was lacking greatly. He said it is really short and easy. I'll post a review when I complete it.