let us, first, look at the definition of sport:
an activity involving physical exertion and skill in which an individual or team competes against another or others for entertainment.
when you take skateboarding and make it a competition, yes, it can be defined as a sport. that mindset, however, is the very thing i personally wanted to avoid by starting to skateboard. i abhor such events as street league and other modern ways of quantifying skateboarding into who earned the most points.
instead, i think skateboarding more closely fits the definition of an art form:
the expression or application of human creative skill and imagination, producing works to be appreciated primarily for their beauty or emotional power.