Well, depending on your view, you might think that American Beauty is kind of a downer. After all, Lester never really reconciles with Carolyn (or forges an adult relationship with anyone else), and right as he seems to achieve some zen and happiness, he gets shot in the head.
Not exactly your typical happy Hollywood ending, right?
Still, though, it's not exactly a sad one. The fact is, Lester does get a chance to feel some happiness and see some meaning in his life—and hey, even one minute of that puts him further ahead than he was when the movie began, right? And Jane, Ricky, and Angela are certainly better off in their own ways (we're not so sure about Carolyn, though—she's pretty destroyed when we last see her).
Overall, we'd say the movie ends on an oddly optimistic note, with Lester reminding us (from beyond the grave) of all the beauty he sees in the world. If you can see beauty in the world after all that… well, that's definitely a sunny end.