I think Naruto was serious with his response, which was that she was "sorta, something like that". He knows she might return his feelings eventually, considering the fact that he she has expressed the desire to not like Sasuke anymore. I think he is somewhat pessimistic about it though. I think we should somewhat consider their perspective (not that it was a joke though) because it would be strange for Naruto to be completely positive about Sakura being his girlfriend after the events of the Kage Summit arc.
He KNOWS Sakura may not have returned his feelings, that's why he said that not everything is going just the way his mom told him. But he said that he's been trying his best, which is what counts.
True. And even without that whole thing, you tend to be pessimistic about others returning your feelings anyways.