I've yet to decide of Mimas is settled or not... probably ought to work on that, really, considering the next chapter. But aside from, you know, magically sustained cities on the moon's surface, there's likely to be no ecological impact, since Mimas is an airless rock, and Saturn is a gas giant. Moving the moon away would maybe have a miniscule effect on Saturn and Saturn's other moons, but overall, I doubt gas giants tend to get much effect from smaller moons, due to their fluid 'surface.'
On the other hand, Mimas appearing over a rocky planet covered in life, like Earth, would probably cause all sorts of problems. Gravitational issues alone would be pretty nasty. And not being a physicist or whatever group of mathematicians studies things like that, I'm sure there's a million and one other things I'm leaving out.
Still. Mimas as a Death Star, while fun to think about, is far too overkill for any of the myriad scenarios and plans we have for the next, ohh... Three arcs. If we were to actually come to the end of those three arcs and begin working on an as-yet-unplanned fourth, then I suppose I could revisit the idea if the opponent we come up with justifies it.
After all, without the superlaser, the Death Star is merely an expensive space station/spacedock. With the super laser, it's a weapon of mass destruction. No Surgical strikes, no finesse. It's a planet killer, and very few problems should be solved with such finality if alternative options exist.
That's what the nukes in the Sub's missile tubes are for.