Not very realistic in what respect? Do you mean because it is needed or the way it is simulated in MSFS?
I’ve never had the chance to fly a real Mustang of course, but I can see the logical/physical need for the differential toe brakes to be applied even if to initially induce a tighter turn. I would have thought the castor design of the tail wheel would mostly result in an aircraft naturally returning to forward motion without the another influence, e.g. left or right toe brake slightly applied or to a lesser extent airflow over the rudder when deflected.