Uh-huh. You don't duck someone by fighting someone better.
When you don't want to risk losing your title you do.
RDA at LW and Edgar at 145 both pose a lot of problems in their own right. Even though RDA is seen as the tougher matchup, a fight with Edgar carries much more risk.
McGregor wanted out of 145 but he wanted to leave as champ. So the champ avoided a fight with the clear number one contender because it was too risky.
If the champ avoids a fight with the number one contender because it is too much of a risk to their title/image, they're ducking.