Third party candidacies tend to do well when the incumbent president or party is so unpopular, that the primary opposition candidate cannot contain all the defecting voters.
This was decidedly not the case in 1988. Reagan's popularity was in the 40s throughout most of that campaign, and up in the 50s by Election Day. Though the public was lukewarm towards Bush, he did not inspire sharp opposition.
In contrast, in 1992, Bush's approval ratings were in the 30s, and there was widespread fatigue with 12 years of Republican rule and a lackluster economy. That was the kind of climate that enabled Perot to capitalize and win close to 20 percent of the vote.
In 1988, with Reagan still relatively popular, Perot would not likely have won more than 10% of the vote - and probably considerably less. He might have denied Bush a majority, but at best it would have looked a little like Clinton's 96 victory, with Perot drawing somewhere between 5 and 10 percent, Dukakis stuck in the low 40s, and Bush in the high 40s, or just about at 50 percent.