Simplicity is a virtue, but it's not the only one. DuDub mIvwa' and Du'arghHa'moH mIvwa' have roughly similar meanings, but they potentially have different connotations because of their different root verbs. The former focuses on the idea that L'rell's appearance has gotten better, while the latter emphasizes that her appearance used to be worse. Since Qov presumably knows that the word Dub exists, I assume she chose 'arghHa'moH deliberately for this disparaging connotation. (I haven't seen the episode in question, but Memory Alpha tells me that Kol and L'rell don't get along real well.)

Also, I'm not sure that a Klingon would necessarily consider Du'arghHa'moH to be substantially more complex than DuDub, since they're well-accustomed to verbs with multiple suffixes.

maghoHmeH!