I don't think osk is a fan of microtransactions. He sticks by the Free-to-Win tag, having been inspired by osu!'s model.
Now, that being said, in-game currency and being able to use that for skins is an entirely different story. You can just play the game passively and generate coins (or lock it to boss mode if you're so cruel) until you have enough coins to buy a new skin. You know who already does that? Tetris 99. What does it cost you? Time. That's it.
To an extent, I'm also anti-microtransactions, but this is a very reasonable ask knowing other games (of varying genres too) already have an in-game currency system that doesn't need microtransactions to keep its playerbase happy.