If there's anyone who could have made America "great" it's him. Of course I'm not American so I know I'm butting in where I don't belong..
But Carter is far and away the 2nd best ex-president the US ever had (Geo. Washington gets the 1st place nod for choosing to set the custom)
But forget all that and examine it on the merits: As a start, he inherited the post-Vietname military, a shambles already, OPEC embargos, Iran, an America that had lost great credibility in Vietnam. He did Camp David, the most successful and effective security action in the Mideast to this day; began deregulation (yes it was him, not Reagan); put Paul Volcker in the Fed and signed the relevant legislation to get inflation under control; implemented the new all-volunteer military; was right about energy policy, ...
The USA switched to a volunteer force 3 years before Carter was elected. What he did do, was re-establish draft registration. Not something one should brag about.