For 30 years Dan Parker, 62, sports editor of the New York Mirror, has kept a loving and vigilant eye on boxing. No one in the daily newspaper field has been more pungently critical of the forces that have damaged the sport. This month, in recognition of Parker's late 1955 series, "They're Murdering Boxing," the Mirror received the N.Y. Newspaper Guild's Page One Award for crusading journalism. It was the second Page One Award for Parker, who was honored a few years ago for campaigning consistently for decency in sports. Says Dan, a 6-foot 4½-inch heavyweight: "I think my series has contributed toward cleaning up boxing because it came at a critical time," i.e., after the appointment of Julius Helfand, New York's aggressive new boxing commissioner, set the stage for long-needed action.