彼はつまらない男だ。

つまらない: insignificant; unimportant; trifling; trivial; worthless

In this sentence, no. Your translations are not つまらない男 but should be 取るに足らない男.

彼はつまらない男だ = He is a boring guy. Sometimes a successful person has no sense of humor. You may respect such a つまらない person, but you would not like to hang out with them. You can’t describe them as “insignificant”.

つまらない means “insignificant” in the context of, for example, “please accept my small gift” = つまらないものですが、どうぞお受け取り下さい。

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