じみた

じみた goes straight after a noun and says something has the air of that noun about it, and that this is a bad thing: "childish", "preachy", "theatrical". It is the た-form of the る-verb じみる and is used mostly before another noun.

As a predicate it takes きごう〜ている, which describes how something is now, or きごう〜てくる for a change that is setting in.

じみた or っぽい

っぽい also means "-ish" and is the everyday word, often neutral: こどもっぽい子供っぽい can be fond. じみた always criticises, and it sits in a small set of nouns: こども子供, せっきょう説教, しばい芝居, としより年寄り, しょたい所帯, きょうき狂気. It does not attach freely to any noun the way っぽい does. じみ地味 ("plain, drab") is an unrelated word that happens to sound alike. ×地味じみた is not Japanese.

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じみた belongs to writing and fairly educated speech. In conversation, きごう〜っぽい or きごう〜みたい is the everyday choice, with the criticism carried by the context.