Graffiti is a fucking mess anywhere not youth specific or out of sight. There are some impressive pieces of work I have seen done around and about, but it would be worth so much more on a canvas.
I think it can be. Right near where I live, there is a wall on a store where people go and do graffiti. Some of it is just tagging and is sort of ugly, but people who come afterwards cover that stuff up. Most of it, however, is very detailed and beautiful. Some of you may have heard of a guy who calls himself blu. He made something called muto, where he painted graffiti on a wall, took a picture, painted over it, painted the next thing, painted over it, etc. I think it's an amazing example of artistic graffiti.
But, any way you look at it, it's illegal and vandalism. It can be very nice to look at, but at the same time can get people killed. I saw something on the news where someone had tagged a warning sign that said to slow down for a cliff, and the dude didn't see it and drove off of the cliff and died. His daughter in the back seat died as well I believe. I could be wrong on the details, but it was something like that.
To answer your question, it could be art, and it can not be.
I have seen the hospitals in my city with a large painting of Jesus healing some people painted on the side of the building. That's good graffiti. The people at the hospital probably had someone to do it.
On the other hand, bad graffiti is involuntarily drawings/sprayings on someone else's property. I have seen gang names that have been shown on the news, on an abandoned gas station wall.