This tag is usually attached by the viewers who deemed the artwork to be "a work that reflects the darkness of heart" or "a dark themed artwork drawn with eminent art skills".
Similar to other ranking tags, it is mainly attached by the viewers who deemed it worthy of this tag, and it is connected to tags like Beautiful or Futsukushii, hence it is almost never used to tag one's own work.
Why "Depressingly Beautiful"?
In Japanese, this tag is labelled as 鬱くしい. While it is not a valid Japanese word, the kanji swap between the word depressing in Japanese (鬱, utsu), and beautiful (美しい, utsukushii) fits the purpose of the tag perfectly, as both the kanji is interchangeable and sound the same, even if it is not a valid word on it's own.
Since there exists no possible nor suitable wordplay for the English word, "Depressingly Beautiful" is used instead.