More and more questions are related to tikz/pgf and not tex in here: should not the creators of this site think of a dedicated tikz/pgf forum?
I disagree. Firstly, with regard to the "pollution". I'm not usually an advocate of tags, but this is one case where I think that the tag system works. TikZ/PGF questions are easily identifiable as such and usually correctly tagged, so using the ignore or follow features of tags should actually work here.
Secondly, with regard to having a separate forum, I don't see any advantages to this. The main point is that there is no clear separation of users. I answer quite a lot of TikZ questions, but I ask quite a lot of non-TikZ questions (and sometimes answer if they're easy enough). For both, I'm in the same "mental state". If I see a TikZ question and I'm thinking about TeX-related stuff, then I don't have to "shift gears" mentally to think about the TikZ question, and vice versa. On the other hand, if I was browsing MathOverflow and came across a TeX question then I would have to do that mental shift, showing that TeX questions don't belong there.
Lastly, I think that TikZ/PGF questions benefit the site. I don't think it is big enough to lose a large section of its questions, and they do draw people here. They also provide a showcase of things that might just make dull papers a little more interesting to read. Think of it as a "Look what I can do!" (in the nicest possible way, of course).
If you mean with forum a site like TeX.SX (which is not a forum but a Q&A site) then you should know the following:
All stackexchange sites need to be officially proposed on http://area51.stackexchange.com/ where they need a certain number of supporters to get the site into the beta stage. Once the beta site shows enough activity it is elevated to a full site or shut down otherwise. It isn't very easy to get so far if you don't have the right amount of supporters and your proposal can be closed as a duplicate similar to questions here. Because TikZ/PGF is for LaTeX and there is already a site for that, I can basically guarantee that such a proposal will be closed quickly.
There is already a "PGF/TikZ forum" in form of a mailing list which is accessible under http://old.nabble.com/PGF-and-TikZ-f3582.html. Also http://www.texample.net/tikz/resources/ is a dedicated TikZ website.
(After the migration, I rephrased (and also changed my point of view))
At first I agreed with the statement because there are several pgf/tikz post in this Q&A which are not stricly on TeX; but then, reading all other answers, I realized that the good use of tags may itself be a good alternative to a dedicated Q&A, as suggested in the question, which will indeed add a bit of noise to the whole system.