Cameron Crowe had a notoriously tough time trying to decide on a title for the movie. The director's cut of the film is simply called Untitled, which was the working title of the project. DreamWorks wasn't having any of that, though, and pressured Crowe to find a better title.
After some serious brainstorming, Crowe had a few decent candidates, but we think he ended up making an excellent final choice (source). Almost Famous refers at once—in the words of Lester Bangs—to Stillwater's status as "a midlevel band struggling with their own limitations the harsh face of stardom," to the lifestyle of Penny Lane and the other Band Aids who are a part of the inner circle but aren't actually famous, and to William's own emerging identity as a rock journalist, still in many ways on the outside looking in.