The only MUPS that I ride are not during my commute, and usually the worst thing on them is the piles o'dog crap. If I were to stop to clean up each pile, I would never get anywhere. The ones that REALLY piss me off are the ones directly in front of the "Please clean up after your pets" signs. The signs have bags and handy attached cans, so the only explanation is simple laziness.
There is usually a small bit of vegetal matter on the trail, but that will blow away, and crossing under a couple of bridges after heavey rains there will be mud on the trail, but the city usually uses a skid-steer loader to clean it off relatively quickly....within a few days of the rain.