Cesar, "professional" is not a characterization you can make without some kind of context, and the money involved is not really relevant. There are career DJs who by literal definition are "professionals" yet, it does not speak directly to their conduct and whether they truly act professionally. Likewise, there are plenty of part time DJs who do very few events but manage to conduct themselves each time with very professional conduct.
By and large the money is irrelevant. DJing that is focused exclusively on one type of event (weddings) or one type of need (celebration/entertainment) comes with some very serious constraints that can't be overlooked. For one, it means the work is almost always going to develop on weekends - and mostly Saturdays. That right there dictates a part time workforce as the most efficient way to address the demand. If you were running a multi-op dedicated to this limited field of events you would specifically want to hire mostly part-time DJs since there is not enough demand to justify full time wages and benefits across the board.