After Quaid starts the reactor and Mars gains an atmosphere, would the temperature be comfortable enough for human habitation?
According to the film's official novelisation, the temperature (post-event) was chilly but not intemperate.
They recovered some of their strength and sat up. They became aware of the chill of the air. It had burst from the mountain warm, but as it expanded it cooled. Snowflakes were falling on them. But the ground itself was warming now, as the heat of the nuclear reactor spread out, so they were merely cold, not freezing.
Elsewhere on the planet might be rather colder but since everyone on Mars seems to live on or near the mountain, it doesn't matter that much and given that Mars now has an atmosphere (to absorb sunlight) it will undoubtedly warm the surface beyond the reach of the reactor's warmth.