I'd hesitate to read into four lines on a flag that could easily just be minor detailing.
That said, Ruberiot plays the part of the Hanged Man perfectly in the Song Day fiasco.
The card traditionally represents enlightenment through self-sacrifice; undergoing some trial to gain wisdom or fulfillment. Ruberiot's goal in writing Star's song was to fulfill himself as a songstrel by making a truly creative and new Song Day performance, and must have known that forgoing tradition was taking a great risk, even without going about it by antagonizing the Royal Family. Ruberiot's "sacrifice" results in a great deal of knowledge being revealed, namely the loss of Glossaryck and the spell book being learned by the people of Mewni and the Magic High Commission (a revelation that probably saves Moon's life), and Star's crush on Marco is put in the open, forcing them to come to terms with each other in "Starcrushed."
Addtionally, the Hanged Man is often seen as forming a relationship or bridge between worlds, suspended between Heaven and Earth. While it's not religious here, part of Ruberiot (and Star's) goal was to present/expose Star as being just as flawed as any ordinary Mewman rather than an exalted perfect Princess. More literally, the finale of the performance is all about Star's relationship with her "Earthly prince," Marco.
Lastly, the Hanged Man itself depicts a man suffering an unusual method of hanging, used to punish traitors. Shouldn't have to explain that one much.