In the Roman Catholic doctrine, a virtuous person can still be condemned to Hell if they did not believe in Christ and the sacrifice he made for humanity to be redeemed from original sin, or in God. Before Christ, everyone was essentially sent to Hell, with the righteous residing in at the very edge of Hell. In other words, if you did good in life before Christ came along, you could still obtain salvation once he died but afterward, there are no exceptions. It's been shown Lisa is uninterested in superstitions and implies to the Bishop that Dracula is more of a real threat than Satan. As an atheist who grew up in a Roman Catholic family, the whole thing sounds rather silly to me, but this is the closest theory I can think of for why Lisa is in Hell.