As someone who isn't in the arts field at all, so please bear with my probable ignorance: why do you not want to let them coast through?
Each class has a set of requirements that need to be passed and the pupil's level of performance at it graded; if someone is gifted at it then they're already at a high performance level.
To put it in a way that's more related to me: I'm a software engineer and lets say that I decide to go take some first year software engineering classes again, particularly the ones related to learning how to program: I will blow through the classes in 1/5th of the time while playing a game on another monitor and still do very well. Shouldn't I get a high grade despite the fact that I put a lot more effort into my invasion of the Soviet Union while playing Japan on Hearts of Iron while I was also developing?
However, teaching people life lessons in the form of a permanent black mark on their record isn't exactly for their own good. For better or worse, those high school grades do actually matter for college admittance.
Yet, maybe they did me a favor. I was able to underachieve to my heart's content at a college full of dummies. Who really needs a prestigious college anyhow.
It's important that you tell people how much effort you spend on things when they don't see it happen. It's true in so many ways; you learned it early.
Also I'd add that something like art class is particularly hard to grade because you don't want to let the naturally gifted coast through. You want to hold everyone to higher individual standards. The teacher may have thought this must have been easy for you and if you put in more effort it could have been way better. Not always right, but sounds like that was the info available to him.