The reason why this is an important question is that you may feel the need to heat food on your plastic dishes within your mess kit. This would not come into play if you were backpacking and hiking, but you may keep a microwave in your camper. A lot of the dishes that are included in these mess kits are crafted from plastic (such as a thermoplastic polymer like polypropylene). As such, sometimes the only feasible way for you to microwave food is to place the food on these types of dishes. However, this should never be done to microwave your food.
Here is the reality of the situation; plastic is not meant to be placed in a heated environment such as a microwave. What happens is plastic ends up releasing disrupting chemicals when it heats up. Also, if heated for too long, any plates, cups, etc., can end up either warping or melting altogether. Err on the side of caution and try not to microwave plastic. But, if you must, do so in small increments to prevent the plastic from getting too hot.