As one of the skaters I appreciate your concern. There are a number of us who ride there in the evenings a few times a week that are trying to encourage less trashing of the park. We're making some progress and the vision for an improved park is inspiring more people to care about the space.
The skatepark is for more than just the guys and gals that leave their garbage behind and I'd encourage you to think about all the positive things that come out of there. On our end we'll do our part to keep it cleaner. Sorry on behalf of our fellow skaters.
Tl dr: When the fence comes down we will be able to get the garbage behind the corner bowl. Sorry that the folks at the dog park have to look at that. It is nasty.