Is using the hydrocortisone 1% cream for the heat rash a good idea? I used it for this heat rash on my right breast, but it's healing up well.

Heat burn . It is ok if the rash is due to inflammation only due to heat with out infection . Check with a physician to make sure there is no associated infection.
OK. As long as it works, it is ok. If you have huge breasts and sweat a lot, be sure that you do not have a yeast infection which would respond better to an anti-fungal cream and anti-perspirant although cortisone cream may mask the rash transiently. See a doctor if this keeps on coming back.
DERMATITIS. It is antiallergenic and antinflammatory steroid cream good for minor skin irritations.