I'm fixing to launch one, and I was thinking about starting on Derpibooru but I noticed Ask art (to Derpibooru users) doesn't seem very popular.
Even if you weed out all the dozens and dozens of them in general, just for Derpibooru specifically seems not supported (that I saw). Is it because those interested just go to Tumblr? Derpibooru users just here to get loads of art, not ask questions? Though even DeviantArt gets several.
As such, for now I decided to refrain from annoying users with another "Ask Me" blog and just post results of the questions from elsewhere.