Was Dennis Hopper's character based on a real life assistant coach?

No. Dennis Hopper's character of "Shooter", who is the town drunk and father to one of the players, is entirely fictional. It is a little ironic (or maybe not so ironic) that this fiction-based performance resulted in an Oscar nomination for Hopper, which was the only acting nomination that the film received (Hoosiers was also nominated for Best Original Score). In real life, Marc Combs and Clarence Kelly were the assistant coaches. Neither of them were drunks.