Gotta say, I don't think even Yuuka would like Flowey once she gets to know him. He may be a flower, but he is also an unrepentant douchebag who has no problems with killing children and monsters alike on a whim. Sure, Yuuka may kill people every now and again, but they usually shoot first and/or do something stupid like screw with her flowers. Besides, he'd probably shoot some of her flowers for the hell of it.
What if Yukari and Koishi got their powers switched?