The game looks amazing and the level of polish in the menus is top notch! Now, while the gameplay concept is a good one, I feel that there are some design tweaks that could make it less cumbersome, mainly the way that the player interacts with the cards. As of now, the moment-to-moment gameplay requires a lot of UI management, since the player has to swap between cards after every use if they want to get the highest DPS. I would suggest that the player cycles between cards automatically after each use, eliminating the constant need to press the swap button. And if you want the ability to stay on one card, maybe there is a button that locks the selected card until released? I also noticed some smaller issues, such as the lack of sound effects and visual issues with the character creator when playing the game in my browser, but those should hopefully be easy fixes.
Overall, I would say that this is a surprisingly polished game for a game jam! If you decided to take this further, I believe that it could easily be made into a commercial release. Great job!