can this be used under a covered porch

Yes it can be

Probably not the best product to use in a damp location, even if it will never be exposed directly to rain or water. I would suggest a stucco joint product especially made for exterior application. Even though it may not be directly exposed to water, the moisture will probably cause problems in the long run. There are joint compounds made by USG and DAP that are for exterior applications. These products can provide a smooth surface also. They tend to be more expensive, but in the long run using the best product for the job always provides better results. Also, never use regular drywall outside. The moisture and humidity will cause it to bow.

Depends on how solid the structure is, How much weather the porch will get inside, and on fast the temperutre changes in the area you live. Mud can crack over time if the room is not taped, major temperuture changes within hours and if the structure (porch) is not solid or prone to shaking.