Stories from books will never be fully represented by movies. I've taken the outlook that movies based on books are only for those people that have never read the books.
Or for those of us that go to the movies because we've read the books, its to receive a visual interpretation of some of our favorite scenes and people, so that when we go back to revisit our stories again, we find them enriched.
So, take that visual of Tonks, with the way that actress portrayed her with that gamin grin and go-to attitude, and reread your books so that she lives again.
That's what I've had to do with Luna, who after the main characters, is my favorite character.
Hm, good question.Well, I think they centered more of the movies in Harry's/Voldermort's Point of View because they were afraid the many meeting of the Order of The Phoenix(group, not movie)would bore viewers. (Even though I found them quite amusing)Also, remember, the movies aren't quite the same as the books, and they didn't have enough time for some of the scenes she was in.
I wish she showed up more in the movies as well; she's one of my favorite characters.
But I'm thinking it was because the fifth book is the longest book of them all (even longer than Deathly Hallows!), and there is so much information. There is so much to get in there, from Harry's trial to Harry seeing into Voldemort's mind to Luna's appearances (she's probably a more important character than Tonks, and she does a lot more, so it's important they show her a lot) to Umbridge's rule to the Order to Dumbledore's Army and the Room of Requirement to the Department of Mysteries, and there just isn't much room for a lot of other things. Tonks gets a few appearances, but they are very short. But if anything, I wish they would have included more from the Department of Mysteries scene. It was one of the best scenes of the entire series, I thought, and they rushed it so much in the movies, and they changed everything :( But back to the subject (sorry about getting side-tracked), the few appearances Tonks does make all show her personality quite well (well, at least in the fifth movie. In the sixth and seventh pt. 1 and 2, I felt she was too melancholy. She was supposed to be more fun, like she was in the fifth... but that's just me XD)