If you've watched Six Feet Under, you know that screenwriter Alan Ball is a master of black comedy and infusing drama with humor. You can definitely see those qualities at play in his screenplay for American Beauty. Just like in Six Feet Under, the whole topic of death is front and center, but there's lots of humor along with the morbid stuff.
Ball freely admits that his own background feeds into his art—for example, he thinks his preoccupation with death has a lot to do with witnessing his sister die in a car accident when he was thirteen. Unfortunately, he's no stranger to profound tragedy, and he brings that special awareness and perspective to the theme of death.
Also of note: Ball is openly gay, and you may have noticed that the topic of sexuality and being closeted comes up a few times in American Beauty, both in positive contexts (for example, Jim and Jim, the friendly couple next door) and negative ones. For an example of the latter, we have Ricky Fitts's father, Frank, who expresses a whole truckload of homophobic thoughts, all of which turn out to be a cover for some same-sex leanings.
Turns out, Ball believes that his father may have been gay (and closeted) himself, and so it seems this particular plot point may come from Ball's biography as well.
Coming from television, Ball was no stranger to having his work picked apart and changed when his stories went from script to screen, and American Beauty, too, underwent some revisions in production/post-production. For example, there was originally a murder/trial plot that didn't test well with audiences, so it was eliminated.
And sure, it's kind of commonplace to view a film's director as the "author" of the film—but given that Ball is nearly always front and center in discussions of American Beauty's vision, we'd say he was pretty influential in the final product, perhaps more so than other screenwriters in this day and age.