After I mulch mow, I can still see brown leaf shreds on my lawn. Is this a problem?

After mulching you may see leaf shreds both on the surface of the grass and in between the blades of grass. But they should not be of the density or size (under 1" square) that will block light and/or smother the turf. Over the winter, those leaf shreds on top of the grass should fall into the root zone soil, as well. There, they will decompose naturally by mid to late spring.

Lawn showing shredded leaves (on right)
after one pass of mulching mower.