Hi!
I know the area fairly well, worked nearby for a while. Île de la Jatte was made famous by Impressionist painters, but nowadays it is nothing special, and really not convenient for sightseeing in Paris as you correctly guessed. Also, there are few shops and restaurants out there.
If you're looking at other places in the area, the "mainland" of Neuilly is already much better, especially if you are close to metro line 1
I've got a degree in art history and so the notion of seeing the places many Impressionist painters painted is well familiar to me, even though, it has drastically changed since then. (Art Institute, Seurat, La Grande Jatte, Asnieres, etc.). In fact, there appear to be a lot of modern concrete buildings (hotels??). I'm not even sure what the "le bord de la Seine" (ou berges...) are like, whether they would be worth walking along. There is a busy two-way street that runs along it.
The island itself, you'd think would be covered with trees and wooded, but that's more like wishful thinking on my part. It's probably nothing like the other islands further down, Ile de la Cite, Ile St. Louis...
But it would be nice to be outside of the major tourist areas in the inner, central arrondissements (Marais, Montmartre, Montparnasse, 5th, 6th & 7th, etc.), all of which I've spent time in. I still like walking through the Tuileries, but the Latin Quarter is a bit much for me (now).