Used 12/2WG NMB wire for internal recessed LED lights, is that bad?
Putting 12/2 on a 15A circuit is no big deal -- the only problems with using a bigger wire than required by ampacity are the physical difficulty of working with it, and the potential for overstuffing boxes.  (If it's too big to fit the terminals on your devices, that's no sweat -- just wire-nut or Wago on some pigtails of the fattest wire the device can take.)

In fact, sometimes putting bigger wire on a long circuit is practically required (it's merely a recommendation in the NEC, but some sensitive equipment hates brownouts....) to keep the voltage drop from becoming excessive. (NEC recommends 5% max over the whole circuit -- it's usually kept to under 3% to allow for extension cords and whatnot.)