Review from a long time (18+ years) casual player of multiple TCGs:
The Outfit Abilities weren't really clear. I suggest mentioning them after cards, because I thought at first that the hat which is pictured was a card. Originally I was thinking that you build up Mana to play cards, but actually you play Outfit Abilities with Mana. I suggest adding info that you can only use one Outfit Ability per turn. Clarifying that attack cards need a matching color Energy in the hand to play them, would be good. My first couple of hands had no Energy but I did have Mana and so only the discard pile was highlighted and I was wondering why tossing a card wasn't dealing damage.
I suggest mentioning in the Tutorial that when you deck out, and the Discard Pile is shuffled back into the Deck on your following turn, that cards you draw on the next turn will be unable to be played that turn and that the 10HP of damage is to a random Outfit piece.
Please add a way to read cards effects in the Discard Pile by hovering your mouse over the card. I also thought Mascot would let me choose which cards, instead of them being random.
The drag and drop system doesn't work well. While I do like the ability to sort your hand, sometimes I would try to drag a card and the mouse would desync from it, so while I could get the cursor over just fine, the card wouldn't be able to make it all the way for me to drop it. (Most noticeable when attacking the Hat with a card from the right side of my hand, but could happen anywhere.) A click to select and click where to attack afterward (or clicking the discard pile after clicking a card to discard it,), or double click to play a card would be really nice. Most cards only target one specific piece of clothing, a random piece, or all pieces, or is a non-targeting ability card, and so double clicking should just play those cards. The only time a selection confirmation should be needed by the player is if a card has a choice on which piece of clothing you damage.
Fun idea for a game, and the only thing I would add to it are Counter cards, which would prevent the opponent from playing a card if your Counter card meets the condition. These cards could either require their own energy like most other cards, or have a special Cost instead. Adding Counter cards, would give the game more direct interaction with the opponent instead of the game just being the first one to deal damage the fastest wins.
The characters and their expressions are cute. You clearly like your OCs. The VAs also did a really good job. The music is good too.
I ran into a bug in the demo where on the hardest difficulty the AI either wasn't ending it's turn or I was unable to start mine. Happened on Stella first then on Arc and after that clicking end match gave a JavaScript error followed by the game becoming unresponsive.