Remark and comment are synonyms in this context.
A comment usually refers to what has just been said, what has happened.
A remark is more spontaneous.
A humorous comment could also be witty, and a witty comment could also be humorous. But witty and humorous are not exact synonyms.
A witty remark would have clever use of language, or something interesting, some insight about human behavior, relationships, something clever, usually stated briefly, in a few words.
Humorous just means funny. Humor is what makes you laugh. Jokes. The unexpected, the ridiculous, the incongruous.
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