No, it's not. People cite "informal" sources all the time. This only really matters on the receiving end of the citation. I.e., if lots of people are citing yo

No, it's not. People cite "informal" sources all the time. This only really matters on the receiving end of the citation. I.e., if lots of people are citing your blog or forum post, then it can be a good idea to take your content and put it in arxiv or similar. Just so that the citations "count" for you in the academic setting. And similarly, it's a bit rude (or at least odd) to cite someone's blog post when there's an accompanying published paper that describes the same work. But citing informal sources where no formal source exists is not (and afaik never has been) a problem.