Interestingly, not as bad as it is to heat formula in a microwave. Microwaving milk alters it's molecular structure and we don't fully know how. For breastfed babies who recieve occasional ebm this isn't a huge issue, but for some babies their entire diet is microwaved formula and this is a big issue. Microwaving alters the immune factors in breastmilk - but formula has no immune factors.
It's quick and easy to heat the bottle in a jug of hot water.
Warm regards
barb