Hopping in here to mirror what Keith said, built a huge park, went over every inch of it and couldn't find near enough litter to cause a 50% litter is disgusting rating, closed my light line and the litter problem disappeared. SOMETHING about the light line is causing issues with littering.
As previously stated, the light line causes (or at least caused--might be fixed now) all janitors to teleport to the park entrance so they don't clean other areas of the park. That is (or was) a known bug linking light lines to litter. Could be there's some other connection between the 2, or that any fixes only solved part of the problem.
HOWEVER, you can have the exact same situation even WITHOUT a light line involved. That is, a huge percentage of peeps complaining about litter yet you've got janitors galore and can only find 1 piece of trash in the whole park. This happened to me recently, with no functioning transport rides at all in my park, and it was because of an error on my part. I had accidentally left a small gap between 2 adjacent janitorial work rosters. This gap had a bin, but eventually the bin got full and overflowed, leading to the complains.
At this time, I also discovered that the percentage of peeps complaining about litter is misleading about the true scale of your litter problem---it usually greatly overstates the case. This is for 3 reasons:
1. Every peep individually complains about every piece of litter he sees, so 1 piece of trash creates complaints by each peep in each group that walks past it.
2. Litter persists until cleaned up by a janitor.
3. Every peep individually will complain about the same piece of litter again if he walks by it a 2nd time.
The result is that you get many duplicate complaints about the same piece of trash. In fact, in my case, EVERY complaint was a duplicate except the 1st one. But this single piece of trash was on one of my main paths, so most peeps in the park passed by it at some point, often more than once. Thus, it didn't take long before my percentage of peeps complaining about litter was like 75%. All for 1 piece of trash, the existence of which caused me to examine my work rosters and fix the gap, and I haven't had the problem since.
But you know, just because nearly everybody was complaining about litter, it really didn't affect anything. That was usually the only thing most peeps were complaining about (other than queue length, of course), so it was just a passing thought that occurred to many. Most of the time, they were thinking happy thoughts. Thus, despite the "popularity" of complaining about litter, it didn't impact my overall guest happiness, park rating, etc. Fixing the problem likewise didn't have much effect.