Um, depends. You'd need to know the specific condition that's causing it to say with any certainty. I think there probably isn't one in the writers' minds when they come up with shit like this, just "hey what if".
It's interesting in that she's clearly an adult with a disability/developmental disorder living independently etc, it's slightly problematic in that she's portrayed as a villain (though that's part and parcel of the source material/BtAS was if I recall good at portraying people with disabilities living non-villain lives, shying away from the full on Dick Tracey style of disability=evil storytelling; some of them even changed their ways).
Anon is correct in that developmental disorders of puberty can be offset (if not entirely overcome) by hormonal treatments. For most people it's little more than height that's an issue; for someone who never underwent puberty, you could fake it with treatment which would cause them to experience growth spurts etc etc, or you could leave it and they'd remain childlike. Since puberty also affects neurological development, as adults untreated they might score pretty badly at some psychometric tests. Undergoing treatment and then stopping it (at completion) isn't a big deal - for females it's probably equivalent to menopause - but undergoing it at all means the body would look at least partially adult.
In Baby Doll's case if she didn't undergo puberty naturally/artificially, then she wouldn't have adult sex characteristics (primary or secondary), but their neoteric forms as of the point at which growth was arrested. Likewise, if we're talking about growth arresting significantly prior to puberty, then neurological development also arrests there - she has the mind of a child, can't do abstract reasoning well, etc etc. depending on the stage.
If on the other hand she just has a form of dwarfism, then she'd fucking love gold and do anything to get it, including stealing, but otherwise be adult in form.