1. There's Feral Ghouls, though they're kind of corpse-like, it's easy to see that they lack any sort of female parts. NPC ghouls still have the appropriate parts, but Feral Ghouls don't. So it's not like they fall off when they mutate or something. Where's the lady Ferals?! (this goes back to Fallout 1, even. There's not a single female ghoul to be seen in Fallouts 1 or 2!)
2. Super Mutants can be female, though they all appear universally male. It's been like that forever, so nothing we can do about that.
3. Trogs are naked, but lack any sort of visual clues. Their faces look fairly male, though.
4. Swampfolk are exclusively male. What, don't they have lady hilbillies? Could be an inbreeding joke, actually. Their mothers are their brothers.
Hell, I don't even remember seeing female Slags in Fallout 2.
Anyhoo, the point I'm trying to make is that there isn't a single feminine mutant creature out there. You can be feminine and scary! I mean, as long as the end result isn't Boobarella, I mean the Broodmother from Dragon Age. Something like http://www.sammageonline.com/images/charpics/illyria5.jpg (okay, technically a primordial demon that possessed an adorable Texan physics genius, but whatevs) would be pretty awesome.
So please Obsidian, give us some feminine Ghost People!