Not for T-Mobile’s current 5G deployment. Only Verizon requires a new SIM card.Yes, if you want to utilize 5G.
Since T-Mobile does not charge an activation fee like ATT and Verizon, I’d still recommend activating a new SIM card. Won’t hurt a bit…!
T-Mobile is deploying another technology than the current 5G mid-band frequency called “standalone 5G” - a poor name in my opinion! - which is not really available anywhere and has no coverage map, and that will require a new SIM card. Until it’s deployed, the iPhone 12 will work perfectly on 5G with your old SIM card.