Yeah, if you're finding subversion in Aishiteruze Baby, you're looking too hard. I won't say it's all surface, but that mangaka isn't really one for deep thoughts or elaborate games. She's set out to draw out a triangle where the essential emotional arc isn't romantic, but rather parental. Kippei demonstrates his worthiness by being a good mother to poor abandoned Yuzuru, and thus attracts Kokoro, whose traumatic childhood loss of her mother couldn't be more obviously Grown Up Metaphorical Yuzuru if the mangaka had hung a sign to that effect around her neck. It's about as family-values as shoujo comics get.
Well, ok, maybe it's a little subversive. But only if what is being subverted is the notion of romantic love as the be-all, end-all of narrative. And really, that's a pretty abstract sort of subversion.