Unless the person(s) selling the calendar composed all of the ASCII art personally, the original authors need to give permission.
Copyright issues aside, for the images I wasn't thinking "ASCII art" in the regular sense, where you stick a regular image into a program and it spits out the sequence of colored symbols that best approximates the original image. I was thinking of each month showing how various calendar-type settings . . . such as a charming, quaint little village . . . would be represented in DF. So, where a "regular" calendar would show a bunch of Thomas Kinkade-style cobblestone cottages with thatched roofs, and a little milkman making his rounds in the distance, the DF calendar would be an almost-unedited screenshot taken directly from the game (Adventurer mode would be better for getting sunrises): Mostly black, with some big brown squares, and some pixelated smiley faces, both inside & outside the squares. And probably some green squiggles of dog vomit.
As for copyright issues, I think as long as we don't do anything that would offend Bill Watterson, then we wouldn't offend Tarn Adams, either. Meaning: As long as the artist's creation is not used to make a
profit by anyone but the actual artist, and as long as the merchandise stays true to the artist's original vision, it's all good. (So, if you pay money for any merchandise depicting Calvin and/or Hobbes, you're essentially flipping off their creator. If Calvin is peeing on / praying to something, that's even worse.)
No I have a better idea, why not do a calendar of fan art with a mixture of player quotes/stories. All profits going to support DF dev.
With so much good fan art & Urist stories flying around, that sounds more like a daily calendar than a monthly. 100% of the profits should be donated to Toady and/or used to purchase the rights to
this image (or something equally badass) to be the game's official icon.
[EDIT:] Added the thing about it being a straight-down screenshot. [/EDIT]