Not necessarily that they "grew on you", more like you don't have as much disdain for them as you once did.
I used to pretty much dislike the entire Solosis line because I was trying to raise something in Pokemon White and wild Duosion kept making that difficult, and while I'm still not fond of Duosion or Reuniclus, I don't think I was being fair to Solosis. I like it, it's a cute lil' brain marble! Even the idea behind the line appeals to me, just not its execution. Caught a Solosis in Scarlet not long ago, and wanted to see how far I could get raising it without evolving it at all... before getting distracted by something else in-game. Probably just doing more Blueberry Quests, haha.
I also remember not liking Unown all that much, until I caught a J-shaped one and put it in my PokeWalker so I could "J-walk". It was so perfect for my Pokewalker, besides. I mean, it's still not winning any battles, but HG/SS gave it its true calling, as far as I'm concerned!
For a more recent example, Tadbulb is pretty cute, but I'm not a big fan of Bellibolt. I guess I don't loathe Bellibolt as much as I could, but I'd be hard-pressed to actually use one, and it's definitely up there on the list of my "Least Favorite Pokemon", in terms of design. Maybe in gen 10, we'll get a cooler evo with Electric Surge, like the Politoed to Bellibolt's Poliwrath. We'll see. In the meantime, like I said, Tadbulb is cute and I like it.
Anyone else have any mercy to show for your not-so-favorites?