In The Motion Picture, Kirk does initially have a special Admiral uniform that he changes out of to wear a crew uniform.
Actually, in the TOS time they at least wear medals and ribbons still. During "Court Martial" and then during the briefing scene where all the flag officers meet in VI the various captains, commodores, admirals, etc are wearing their awards. Spock, McCoy and others also wear them with the early dress uniforms on TOS.