There's a right-wing myth that the term "Office of the President Elect" is a propaganda move used by Joe Biden to usurp the presidency illegitimately. To mock this and to spread awareness, they're adding "President_Elect" to their username. The idea is that if Joe Biden can proclaim himself President Elect, then surely anybody could.
If you point out that President Elect is and always has been, in fact, the correct term for someone who just won the presidential election, they draw you in to their nonsense argument about voter fraud and recounts and how nothing is decided until January 20th itself because don't you know Trump is a chess-master genius who's always thinking four moves ahead and always comes out on top.
In practical terms, it's a form of the MAGA red hat. The intent is to signal camaraderie to each other but by changing their usernames this way, all it really does is signal to the world who's dumb enough to believe this crap.