WASHINGTON—Amid collapsing public confidence in U.S. courts, a federal judge has been found culpable of misconduct. The offense? Questioning the ethics of a Supreme Court justice. U.S. District Judge Michael Ponsor published an essay in the New York Times arguing that the display of flags associated with President-elect Donald Trump’s MAGA movement at Justice Samuel Alito’s Virginia and New Jersey homes was a breach of public trust. The piece was unusual because judges don’t typically offer personal criticisms of a colleague in public. According to the jurist assigned to review the matter, Chief Judge Albert Diaz of the federal appeals...