The particle colors on the ult are way too bright and neon-like. Normally this would be fine but Katarina herself is coated in deeper colors and even the rest of the abilities for the most part are pretty dark. When she ults she looks like https://images.app.goo.gl/nvkqWMU7MJpgsGez8
Now onto the actual outfit. To be blunt, she's too visually noisy. League usually makes do with beautiful and simple pieces put into a design, but this looks like they threw everything they possibly could into her design and it just ends up looking quite messy. I would say either remove some of the clutter or simplify what is already there so that it just doesn't look so cluttered. Also the values of the outfit are way too similar throughout the skin. It doesn't have much contrast which makes it it kind of all blend together and look very basic. This surprised as most of the bloodmoons have been pretty sleek. Hope this gets viewed, as a former character design student this is my take on it.
Thank you - Katarina main who owns every skin