It's not that simple, The Hive Queen is only an ally until the corpses stop flowing or her hive grows large enough to outfight and consume everything around it, and Mantis' mind got completely subsumed by her mother. While saving Mantis kills a long term threat, and still gives the option to pop out hive drones with the caveats of not knowing if the original hive queen went crazy due to giving birth to the drones, and knowing Mantis' control of her own would be limited but the mutants are back at square one with needing either decent allies, or a very successful recruiting drive.future queen and ally to your causes,
or let her be a nobody, just another cog in the mutant nutty leader plans.
But the Hive Queen alliance might be the only short term strategic option the mutants have(maybe allying with the raiders is another?)
Also, while I like the hive queen aesthetic and her body is 10/10 , she's completely insane, Fang's sex scenes are more fun anyways. Fang ain't nutty she just very nearly got too bitter and out of touch.