I'm Such a Fool is a tag and also the name of the eighth subtitle episode for Puella Magi Madoka Magica. Supposedly the episode and the quote itself is impactful, especially to the viewers who watched the show.
But however there are illustrations where people used this quote as a meme tag instead.
For this tag, it is usually treated as a illustrations or derivative works that has reality denied related contents, which goes along well with Stable Sayaka related tags.
Of course, this goes without saying, this tag is highly related to Sayaka Miki.