If you microwave jello then the jello will usually melt back into a liquid and may also explode all over the microwave.
I did place some jello in my microwave before and it exploded and made a mess in the microwave so I would not recommend microwaving jello.
The Jello does not really need heated anyway to eat.
You make the Jello by placing it in the fridge to cool it and then it sets up.
At room temperature if left out too long the Jello may eventually turn back to liquid especially if the temperature gets too hot.
Jello is mostly water but also contains meat.