Jackson was a hawk and sincere anti-Communist, but he was also prudent. He opposed the 1983 Lebanon intervention, and advocated caution in 1980 in the aftermath of the Iranian revolution and invasion of Afghanistan.
I don't see him cutting off aid to Nicaragua, but he could demand a lot of reforms by Somoza to get that help. Furthermore, the Sandinistas were not uniformly Communist. In common with most Communist activities, they always made allies with non-Communist reformers to show a broad front, and then abandon/push out those elements later. Presumably Jackson would see through this faster than Carter, but there are still elements in the Sandinistas that could be worked with initially.
I think the situation in Iran would be similar. People did not foresee an Islamic Revolution. He likely would have acted similarly to Carter (try to build relations with democratic reformers friendly to the US) until the hostage situation happened. The Shah was a valued ally, but his human rights abuses were a complete embarrassment.
In both cases I think you could say he maybe might handle the transition better and might have avoided the need for Reaganesque responses. If not, then the Reagan-like policies are implemented earlier.
However, there is likely to be a big impact on arms talks with the Soviet Union. Detente would likely end.
Domestically the big change is that none or few of Carter's pro-market reforms happen. Jackson was in the mold of traditional Democrat economic ideas. I don't see him deregulating at all, and I'm not sure if he'd appoint someone like Volcker to the Fed. Stagflation likely won't be resolved.