Will leaf mulch under a lemon tree improve the quality of lemons or is mulch not supposed to be under them?

Make sure there are no insects, spores, molds and etc in your leaf mulch. But lemon trees need potassium, and personally I'd use month-old, dry ground banana hulls sprinkled in a wee trench around the base of the plant, a few feet out. Notice which of the branches are reaching furthest out, and that's the limit of the circumference of the circle (ie don't dig into the roots). Put the fertilizer in, some good soil and water, and wait for sun.