My day 1 purchase 12 mini (October 2020) is currently at 93% which is about right. I never used wireless charging at all, magsafe or Qi charging which has probably helped my battery. Only used primarily 2.1 amp charger, slow 5W charger and sometimes my 18W iPad Air 4 charger (which charges very fast).
Its fine though guys, just replace the battery for about £50 and your 12 mini will be like new again.