Shroom Girls aren't great. They have a full set of fixed skills so there's no variation on how you can use them. They also lack an Armor slot, getting a second Misc slot instead.
What they do get high lust growth, which combined with their passives means they can dish out a lot of lust damage. They are also PoisonImmune. For what it's worth, they appear to count as mushrooms in regards to the MushroomPresence passive that the MushroomCore boss has.
- Obtained through the use of a Mushroom Essence consumable item. The Shroom Girl will spawn with the host's genes and genetic traits.