I mulched around the base of my MG plants; careful to not cover any of the plant above ground. I suggested cutting it because a lot of matted material on the base of the plant will probably not kill it ...... but I suspect it could make growth less robust. A substantial covering of matted plant matter (3-4 inches of leaves) can adversely effect the grass in your front yard (might even kill it) so MG - while tough/hearty as a plant - might struggle if sunlight/air are in shorter supply due to materials substantially covering the plant. Again, good luck. One other thing ... and I have no idea if it makes any difference .... MG likes water but not wet feet, so if the plant has material covering it that can remain moist for an extended period of time and you encounter a wet spring ... is there any danger of crown rot, or other problems like rust or fungus?
e.g., Stem and Root and Rot
Courtesy: Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org
Cool wet soils cause a number of stem and root rots. Roots need to breathe oxygen and will drown just like humans. Soil that stays wet and does not drain has all the air spaces filled with water. Adding organic matter to soil, especially clay soils will loosen it and allow water to drain. Phytopthera fungi will attack seedling stems at the ground level, entering the plant in areas where there is environmental or insect damage. Stems will turn dark in color, become thin and stringy and the plant top tips over. Roots will become soft and mushy