Mismatched is the adjective: it describes things or people that are put together but do not go together. You use it in front of a noun — a mismatched pair of socks, a mismatched couple, mismatched chairs, mismatched data — or after be: "the two players were badly mismatched".

Again, the badness is in the combination. Mismatched socks are both perfectly good socks. What is wrong is that they are on the same pair of feet.

It covers several everyday situations. Things that clash visually: mismatched furniture, mismatched colours. People whose strength or level is too far apart to make a fair contest: a mismatched final, mismatched fighters. People who are wrong for each other: a mismatched marriage, mismatched partners. And records that should agree and do not: mismatched files, mismatched signatures.

Here is the part a dictionary can be too tidy about. Tone depends entirely on what you are describing. Mismatched people or a mismatched contest is a real criticism — something is unfair or unhappy. Mismatched objects can be completely affectionate, even fashionable: a kitchen full of mismatched cups sounds warm and lived-in, not careless, and people deliberately wear mismatched earrings. So listen for what is being described before you decide whether the speaker is complaining.