Dwarf Fortress will always be niche, rejoice in your gaming superiority over the masses!
until multiplayer adventure mode is in, along with railguns
I've said it before and I'll say it again in bold this time. Are you looking?
Dwarf Fortress is no longer a niche game (for large forums)
Ok, let me make a qualification. If you meant the unwashed masses as in people who only use the internet to use Myspace or Facebook, then yes, you are 100% correct. However, if you meant "unwashed masses" to mean everybody who browses popular web forums (excepting Gaia Online) then you are dead wrong. If you meant people who tried DF but quit, then you're an elitist prick.
Just think of it this way: How did you find Dwarf Fortress? Unless you say Boatmurdered (comes from Something Awful, a gigantic community) or 4chan (another gigantic community) then most people are going to say something different from what everyone else said. You can now safely presume that the community of whatever your source is knows or knew about DF at one time.
Let's look at the Wiki. The front page has links to 7 seperate large forums with threads that are each 25 or more pages. Something Awful has even more threads related to DF (They're archieved.) Additionally, the wiki is available in 4 non-Enlgish languages. It was started in earnest early August. That's fast by Wikia standards. The vast majority of wikis are available exclusively in English. Let's take this out as well. DFFD is a file uploading site dedicated exclusivly to DF. It seems small, but that's a big deal. Don't forget about DFMA, another site made exclusively for DF. Most freeware games are lucky to have an offical site and fan forums.
All that aside, however, the vast majority of people in the world do not know about DF. However, I would venture to say a decently good percentage of internet forum users at least know about DF, ~4% if I had to guess. Well, I guess if you included very small special intrest forums as seperate, but I'm referring more to general large forums with discussions on a large range of subjects.
You know what? TBH, I don't think redoing the interface to be streamlined will entice extra donations. Mouse support will help an ok amount. Tileset support will help a lot for the first month but will then do nothing to help donations. An included tileset is the only thing that would really spike donations.
It's funny actually, I can't play Nethack or Angband in ASCII, but I'll play ADOM and DF in ASCII just fine. I bet it's the colors or something. Plus, Toady is really good at drawing things in ASCII, like ballistas and workshops.