Wonder how Cutie marks are like in this universe? Artist Didj provide their own take on the cutie marks: called Aura Marks

As for how gaining your Cutie Mark translates into the mythos of My Little Mages? Well, with all things I like to allow the fans to envision such things however they like, building their own mythos around the drawings. But I was to give my own take on it, I'd say that the moment your destiny is realized, magical circles manifest around you for the first time. These magic circles are the catalyst of each Mages individual magical arts. All Skyborn can fly and all Spellborn can use simple levitation magic, but each Mage has their own unique magic arts, which is where you get the individual titles like Pyromancer or Enchantress. These magic circles converge and form an image. That image is reflective of your personality and destiny, it is called your "Aura Mark". Culturally, Mages like to incorporate their Aura Mark into their clothing. So the tradition is that during the celebration of a young mage realizing their unique magic arts, they are fashioned a new outfit.

Source:https://www.deviantart.com/didj/art/My-Little-Mages-the-New-CMC-570687437

This is a listing of the different lore/wordbuilding background from artist Didj. They do make clear that they allow the fans to come up with their own interpretations on how the world of My Little Mages work.
I wanted to post their own thoughts on the verse for easy access so that those who want to build their own stories on this universe have an starting point.

I go with something like the show, the mark is the result of an epiphany.

Huh, that actually made more sense than my theory