What would cause my 17 year old to have a bad sense of smell? She has no allergies but is frequently stuffed up. It is getting worse over the years.

Smell test! If you take your daughter to an ent, they can do smell testing to get a measure of just how limited her sense of smell is. They can then do a thorough work-up to evaluate for causes. Often this is something simple like allergies, however, there can be rare other causes that should be looked for.
See her doctor. It could be anything from chronic sinusitis, polyps or other growths, irritation (due to smog or tobacco smoke, for example), to having new onset of allergies. She needs to see her doctor for an evaluation.
No Smell. Lack of smell = anosmia. Is she totally unable to smell anything or is her sense of smell just decreased (hyposmia)? Either way it could be as simple as allergies since you mention that she is frequently "stuffed up" but you always want to do what is safest for your child and might want to have her primary doctor make sure that it is not anything more serious before just calling it allergies.