Lol, nothing. He ran a decent presidency, especially post-Watergate when faith in the post was at an all-time low, and was consistently attacked from the right throughout – to the point that Iran did not release the embassy hostages until the day their candidate, Ronald Reagan, was inaugurated, as a final FUCK YOU to Jimmy Carter.

I was a young adult during Carter’s term, and while IMO he made a couple missteps (firing his cabinet, the hostage rescue attempt) he actually did really well overall during the era of severe OPEC gas and oil shortages, and none of what you would call his “mistakes” were based in fraud or corruption. The above is off the top of my head, but I was paying attention and I genuinely can’t think of any big scandals involving him or his behavior.

EDITED TO ADD link to the Airline Deregulation Act of 1978: it had unforeseen consequences, but was a genuine effort to break up the airline monopolies and lower airfare prices. It was not even remotely the catalyst of economic armageddon; that’s just overtly misrepresentative shitposting. Even Justice Breyer had compassionate words to say about it (last section):

https://en.wikipedia.org/wiki/Airline_Deregulation_Act