So what's the difference between calling someone stupid and saying they're acting stupid?

One means you think they are stupid. The other is saying they are usually more intelligent than they are acting in the moment.

One is to them , one is to the action

Yeahhh. Calling someone stupid is just that calling them stupid, stating they’re acting stupid is referring to their actions instead of them. It’s hard for me to explain, because it’s kind of straightforward. Lol

They might just be pretending to be stupid, to irritate or manipulate other people. Or they might be intermittently stupid but not all the time. It's a bit nicer to define the bad behavior instead of the person.

Acting stupid would be temporarily doing something that is out of the ordinary for that particular person.
Being stupid is continuously making unwise choices.

Smart people can have a momentary lapse of intelligence and do something stupid and out of character. A stupid person is stuck that way for life.

One is a judgement on their actions, the other is a judgment on their entire being.