No, she didn't. On the contrary, she got much prettier now. She even beat May IMO, which was the prettiest Ash's companion until Serena cut her hair. Dawn, on the other hand, is second ugliest after Iris in my personal list, sorry to all Dawn fans, but I simply have no love for Anime characters with unnatural hair colour. And her cut was disgusting for me too. I only liked her look a little when she had her hair free, which was unfortunately very rare, or her halarious Dee-Dee look, but she did not keep that, unfortunately. That's my opinion on those two.
Personality-wise, I think Misty and May worked best for their particular sagas. Misty was there to try and tame Ash, something that was not really needed that much in Johto (which is, IMO, also why Iris failed, even though they tried to dumb Ash down in BW). So they came with May, even worst rookie than Ash was in the beginning, to show how Ash progressed already. Dawn brought nothing new to the show, she was yet another Coordinator with already established goal, skilled similarly to Ash at that point of the show, which was quite strange for a rookie and did not help with her character development either. Serena, on the other hand, came to the show as someone, who only went to Pokémon Journey to meet Ash and get rid of the harsh Rhyhorn racing training her mother forced on her. Thanks to that, she got a lot of character development. I only hope she will continue with that development and they will not stall her as they did with May after she got her goal and Dawn after she got over her initial loses.
But I still think the best romantic interest for Ash is Gary :P