As seen above the movie could be considered to have a Downer Ending when he begins pursuing relationship with Autumn because of the implications that he hasn't learned anything from his relationship with Summer and is about to do a Here We Go Again... unless you consider the importance of her different name: Autumn. Which means "change".
The ambiguousness is highlighted by Tom's appreciation of The Graduate which, similarly, has an ambiguous ending that would be considered a downer.
Valid, however, the lead up to this shows him finally putting work in towards his other goals (working as an architect), instead of wasting time in the greeting card company. It implies that while he was waiting for a Manic Pixie Dream Girl to come along and change his life, he's gotten over that and is taking steps towards finding happiness outside of his romantic relationships. If he happens to meet someone along the way, all the better, as he's better prepared for that, now.