My opinion on Sumia has always been somewhere between 'meh' and 'annoyed'. But that's largely because I didn't like how hard the game tried to ship her and Chrom together so not-subtly. Similar characters show up in most of the Rune Factory games, and I don't like those individuals either
The games always try way too hard to make these characters endearing and I guess I just rebel against liking characters that games put twice the effort into making 'likeable', because the traits they give these characters are often immature and annoying to me XP I will pick my own favorites, thank you, I don't need game developers to give me suggestions (especially suggestions that come in the form of bright neon signs saying "I'm cute and weird and that means you should like me!").
...er....but I don't actually hate Sumia. She's a good unit, and now that I've had her and Chrom marry everyone else (trying to unlock all the supports with the help of my sisters), it's always a fun challenge to try and get her the Galeforce skill before Lumia joins the team. Because Galeforce is EPIC XD