They cut taxes and debt-financed war, which forced the US further into debt.
It's also very hard to assume that Reagan being elected in 76 would've avoided the oil-driven inflation at the end of that decade.
But of course, we've decided as a country/media to generally blame Biden for non-policy factors that put the economy on a wild bullwhip ride from 2020-to-2023ish, soooooo... maybe Reagan can deal with getting the blame for the inflation too!