I think Ross Perot was appealing to fringes of the Republican party more than to Democrats.
IN theory Ross Perot cost Bush votes , but this would be a hard to prove theory.
It may be that many Perot voters would have not voted for anyone if Ross Perot were not in the race.
But in this theory Jill Stien would siphon Democrats , so the Democrats stand against her being on the stage for debate.
I don't think the Democrats are interested in experiments that would test this theory.