Why does this Egg McMuffins recipe taste terrible?
For starters, you left off the slice of Canadian bacon. McDonald's cooks their egg more by steaming it in a round mold; they do not fry them in a pan
You might try going to the Serious Eats site and looking at Kenji's recipe for Egg McMuffin:


  Pan toast muffin in butter, fry Canadian bacon in butter, crack egg in lid of wide-mouth mason jar to use as a mold, break yolk, pour water in the pan with egg in mold and put a cover on pan so the water turns to steam to cook egg, drain egg on paper towel, assemble {muffin half, bacon, egg, cheese other muffin half}. No mayo, other condiments or seasoning.