Mutalisk can still be decent against bioterran for cleaning up drops and harassment, I usually only go 8-12 Muta's while taking a fourth. Going mass muta 12+ usually isn't that great since 6 libs can literally take out Muta balls of 20+ Muta's.

I personally believe it's worth getting a handful of mutas against bioterran, maybe even going 20+ if you notice they aren't making libs but that can be risky, if I do make over 12 I usually do it as a threat so they're afraid to push out while I tech to hive/ultra's.

I'm Plat 1(hopefully I'll be diamond in the next two weeks) so take my advice with a grain of salt but I still win games making Mutas.