It's an insult. If someone calls you dim-witted, you would likely be offended. It's a way of calling a person slow, not bright, dumb... ect. If you tell somebody, "You're surprisingly dim-witted," you're basically saying you thought they were smarter than what they really are.

The "surprisingly" here has rather the same meaning as "more than the speaker expected"; i.e. "you're more dim-witted that I had expected". "Dim-witted" means "not (very) bright/clever/smart".

Dim- low light wit- humor with brains Dim wit - low amount of brains Surprisingly dimwitted - an unexpectedly low amount of brains

The term dim-witted is an old-fashioned term to describe someone as stupid or not very clever. Several idioms such as "Not very bright", "Not too bright", and "Not the brightest" all connect to saying that someone is not intelligent or quick-witted (The more obvious ones were people say "He's not very smart" is a prime example of this). The more famous phrases "Not the sharpest tool in the shed" (to being used in the All-Stars song as "ain't the sharpest tool in the shed") or "Not the sharpest knife in the drawer" all mean someone is lacking intelligence; stupid as a quick search on Google. And yeah, surprisingly basically means something unexpected. Although I also like to be informed and I really like to learn in general!