Early game tristana is one of the worst of the ad carries until 6. It's the fact that she's one of the scariest late game ad carries that's the reason you play her. She's pretty much never in range because of high range, high mobility, a position disruption ult, and an attack speed steroid that's pretty much unmatched. Her only downside is she's pretty much pure ad, which means she doesn't fair as well against tanks, unlike vayne and kog who have onhit effects working for them. However, she attacks faster, has a larger range, and has better mobility, so the sacrifice in tank shredding is made up for with bruiser and carry shredding, along with escaping.
She's one of the best, if not the best, ad carry in terms of theory.