What does "feel dumb" mean?

“Dumb” is the equivalent of “stupid” in this context. Are you not a native speaker of English?

Stupid, but a better word in this context would probably be ‘silly’.

I agree with @ibstubro, though stupid in a self-deprecating way, not stupid intellectually. (Self-deprecating means modest about or critical of oneself, especially humorously so.)

Rudd, though at 46 heading rapidly toward middle age, still clings to his Peter Pan image, as do many other of his generation. His manner of speaking, at least on camera, reflects that.

Sometimes I couldn’t achieve something that I clearly know that has always been very easy and accessible for me. I think at those moments I can relate to this feeling.

It’s like having a fever that does not exist maybe? During a fever, you can’t do anything, but you just know you could do them before.

I agree with @janbb and @ibstubro. It actually means stupid but to a lesser degree. It usually would mean foolish and, or ignorant, depending on the context.

Example. If a girl caught her boyfriend cheating, she may say I feel dumb. Not really meaning that she thinks she is stupid but foolish for believing in him.

Example. An adult in an amusement park ride that is meant for children. May say I feel dumb and they may mean silly if no one they knows witnesses it or foolish if someone witnesses it and mocks them for it.

Example, A kid saying I feel dumb after failing a rather simple test. Means they feel stupid.

Example, Someone quoting something they believed to be fact and find out later that the information has changed since they learned it , may say I feel dumb and what them mean is that they feel ignorant.