Zimri’s name is similar to Zemiros. The Gemara says that Zemiros go well with learning, and without them, a person’s Torah is called: חוּקִים לֹא טוֹבִים “laws that are no good”. Although Dovid was full of Simcha'dike learning and tremendous dancing (and even extreme, especially according to his wife Michal who disliked it), yet, there is a time not to sing your learning. The Gemara says that Dovid was punished for saying: זְׂמִרוֹת הָיוּ לִי חֻקֶּיךָ (Tehillim 119:54) Your commandments are to me like a song; this is considered a Zil'zul, if you look at Torah as a mere song.
Although Simcha in Torah is especially important, and in Birchos HaTorah we say: והערב נא "Make it sweet!"; yet, it may be that calling Torah “Zemiros” also symbolizes a lack of Ameilus (toil) and seriousness towards the Torah. Of course, Zimri who was a נְׂ שִ יא ב יִ שְׂ רָ אֵ ל prince in Klal Yisroel isn't a Pashut'a (simple) Rasha, and perhaps his Chisaron was that he took his happiness in Avodas Hashem too far, and which caused a: קַ לוּת lack of seriousness, and as a result he was Nichshal with Cozbi.
A person always needs a proper balance between his Simcha and his seriousness. Like it says: וְׂגִילוּ בִרְׂ עָדָה (Tehillim 2:11) rejoice with trembling. As much as our contemporary Mussar Seforim are loaded with Chizuk, yet, we can't forget Y'mei HaDin and Elul and the Midda of Yir’ah (fear of Hashem). When we do things properly, then we come out happier and safer. There is a Gemara where Chazal were too happy, and they had to break an expensive vase to get people to sober up.
As important as Simcha and a kosher laugh is these days, you lose your whole Chen (appeal) when you go overboard, and: וְכָל הַמּוֹסִיף גוֹרֵעַ excessiveness only messes things up. That's why a Rebbe is so important.
