It is probably that Meteora's led a bunch of lives that leads to her retaining those abilities. It's just doing that with a baby character felt a little too far for me, but to be fair, that's also because I was completely expecting the series to end with her as the half-monster half-Mewman princess, and possibly with the only magic left in the world.
Now that that didn't happen, I'm not as annoyed about it as I was.>>107668275
For what it's worth I was a fan of Eclipsa once too, but she got old fast when she started getting too much screentime and being portrayed as too purehearted/perfect. I also would've preferred her being a villain or more dangerous or her flaws being used to greater effect and with more negative consequences.
In the end I'm not even sure if Mina was the main S4 antagonist, so much as just one antagonist of the season (along with MHC, RACISM, people who don't like Eclipsa, dark magic, Moon, etc.) and reason for destroying magic. She didn't even get a final fight scene really.