Is it a good idea to grind dried oregano with a mortar and pestle?
By grinding it, you are also increasing the surface are of the herb when it reaches the tongue, and you are exposing the raw/inner (bitter) flavors of the herb to the mouth. When cooking with it "un-ground", the cooking process extracts just the oils from the herb, and leaves the leaf in tact which does not taste unpleasant to the senses.

I would certainly use less of a ground herb, but my preference would be to leave it in it's natural dried state, and if you want more taste just increase the quantity. The only exception I have for this is when making rubs, where the fat of the meat (or oil for a bread dip) typically complements the bitter flavor of the ground herb.