The Mars Volta for mainstream audience? I don't think so. Well, it depends what is exactly mainstream. Pop fans who never tried some experimental music probably won't like them.
I needed to have some listening experience to appreciate them. Three years ago I would stopped and call them just crazy bunch. But now I love their weirdness.
Well, I wasn't so accurate. Of course TMV aren't in cathegory like Pink Floyd, The Doors and so on. They are their own cathegory but with this I wanted to say they can have special place in my music library and / or in music history.
sillysillymary thinks mood of their music is angry. I would say it makes your heart beating and it push energy to you in such a way that you aren't always ready for it.
This is great interview with Omar -
Interview: The Mars Volta - ARTISTdirect News