OP, did you complain at the time? If you didn't bring it to anyone's attention, then I don't see the point in posting. The bathrooms are not permanently messy, or even messy most of the time. What you experienced is not the norm.
Unfortunately people can be slobs. Where I work we share a bathroom with the public, so I know for a fact they can clean it, and a few minutes later some slob(s) can trash it. Hopefully you brought it to the attention of the CMs working in the park so they could address the problem. No, you shouldn't have to, but sometimes you need to.
People shouldn't have to be told to flush toilets either, but until Disney has a CM stationed at every bathroom stall, some people won't and the rest of us will have to do it for them. It is gross. But I blame the piggy guests more than I do Disney. It is sad that people can't take care of a basic body function without making a mess.