Toilet flange slightly angled, options
The simplest option, assuming that the flange would not stand too high above the finished floor, is to do nothing. The wax ring used to seal between the flange and the toilet can take up quite a lot of misalignment. If the difference from the high side to the low side is less than maybe 1/4 inch (or even more, possibly) it'll be fine.
I'm not sure that a hair dryer will make enough heat to soften the ABS pipe but a hot air gun would.
It sounds like you're still in the rough construction stage where pipe replacement is simple. If so, and if the angle is really bothersome, cut it out and re-do the work. Mistakes happen and sometimes we have to re-do stuff but that's ok.