Is it bad to be sprayed by a fire extinguisher?

If it belongs to someone else and you're doing it without permission, yes; if it belongs to you, then "stupid" is probably a more apt term for it. There's not a great deal of fun to be had in playing with the now-ubiquitous dry chemical types anyway. Anecdote: many universities require graduate students in chemistry to take a fire safety class. In one I took, the instructor told us that they used to have the old brass pump-style water fire extinguishers in the dorms; they would occasionally during an inspection find one that had been filled with beer as a sort of impromptu keg. The good news was that beer would at least still put out a fire. When they discovered one that had been filled with gasoline, though, they removed those and replaced them with carbon dioxide extinguishers. However, they found those frequently needed recharging: students had discovered you could chill a can of beer quickly by sticking it in the horn and setting off the extinguisher for a second. They were in the process of replacing those with dry chemical extinguishers; in the Words of the fire safety instructor, "The students haven't yet found any way to do anything with those except make a mess, which they have to clean up, so they don't get tampered with much."