I haven't actually seen a lot about the plot for Mythix. But this is what I think about the design.
Bloom- She looks fantastic. I think the colors are original and they've never been used for one of her transformations before. She looks ready for a ball, but not for a battle. Then again, none of them do. The only thing I don't like that much is the wings. A lot of people say they are amazing, but to me it just looks like they took a generic wing shape and added some scribbles and holes.
Stella- I like her boots and the brooches on her dress. Her wings are also a bit new, with nice, matching colors. I just wish they could have done more with her dress! Compared to Aisha's details, she is definitely not the fashionable one in the group here.
Flora- I don't get how anyone can like this! It's horribly plain and you could find a dress like this at the store. Every single outfit on Winx Club is better than this dress, period. The only part I like is her wand. The colors are bright and the shape is not just an anyday flower shape.
Aisha- Hers is one of my favorites. The colors are pleasing to the eye, and they actually put some effort into the patterns on her dress. The shoes match perfectly as well. All I wish they had done was put something on her arms. They look so plain.
Musa- This might have been a good outfit... on Stella. I think it's nice, but it does not fit Musa at all. Aside from that, you can tell that they put effort into it just like Aisha's. They did a great job.
Tecna- I'm indifferent on this one. I enjoy her skirt, but the plainness of the rest is just sad! Her wand is also ugly. Some people don't like them, but to me, the wings just kind of tie the whole outfit together. They're a nice touch.