You can write an article about any term or concept so long as it doesn't violate pixiv's Terms of Use or the pixiv Encyclopedia Guidelines.
The advantage of writing an article with the same title as a pixiv tag is that the article will be linked from the search results page for the tag, making it easier for fans to find and read. If you're wondering about how to title your article, you can consider the proven strategy of using a pixiv tag as the title.
For more details, check the section about how to title an article.