Translation:She is not serious!
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"是……的" is not being used here; "認真" is the word for "serious": https://en.wiktionary.org/wiki/%E8%AA%8D%E7%9C%9F#Chinese.
認真的 is a vocabuary, 的 is a meaningless function word but tells you this vocabuary is an adjective. 的 is put behind in every adjective, but sometimes you can hide it.
I'm native speaker, but sorry I can't tell you when some certain word can hide or not. Give you an example, for these 2 sentences, the meaning is different.
他很認真 Descibe he is a serious person(adjective here), or he fucuses on doing something(adverb here). 他認真的 Tend to describe he means it, not kidding(adjective here)