All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
It is permissible for a Muslim to skateboard if he does not fear that it would lead to harm, as in principle all things are permissible unless proven otherwise.
The fact that most of the people in the skate park are not Muslims does not Islamically prevent one from being there and skateboarding unless there are religious prohibitions such as forms of nudity and other means of temptation and corruption. If this is the case, then a Muslim should leave the park in order to safeguard his religion and honor.
Allaah Knows best.