When cooking or baking is it better to steam peppers or add them raw?
Jalapeno added to a muffin should be minced sufficiently finely that it is not necessary to par-cook it in any way, but you may choose to in order to develop flavor.

When adding larger peppers like a pablano, I like to roast (not steam) them first, to:


Remove the skins
Soften the flesh, as the larger dice may not completely cook during the baking process
Develop that delicious roasted flavor


Incidentally, I find a can of diced hot peppers performs extremely well in corn muffins, and is very friendly for last minute baking out of the pantry.



The heat in peppers is due to a chemical called capsacin, which is found mostly in the ribs and membranes surrounding the seeds.  You can influence the heat level in your muffin by how much of this part you include.

By cutting out the ribs, membranes, and seeds and using only the outer flesh of the pepper, you minimize the amount of heat; by including these parts, you maximize it.

Whether or how you par-cook the pepper before baking it in the muffin has no real influence.