Is it safe to microwave my cat’s wet food to warm it up?

It can be, but the real risk is uneven heating, not the microwave itself. Microwaves heat unevenly, so a bowl that feels warm on top can hide a scalding-hot pocket underneath, and cats have sensitive mouths that will not test the temperature the way a person instinctively does with a spoon. If you warm food this way, stir it thoroughly afterward and actually test the temperature yourself before serving, aiming for lukewarm rather than warm. A short stint in a bowl of warm (not hot) water to bring food to room temperature is a lower-risk way to get the same appetite-boosting warmth without the uneven-heat problem.