In a sense, you could say he was because he was trying to help overthrow his king. But morally speaking, it's pretty hard to argue that Macbeth had any true claim on Macduff's loyalty. So technically maybe he's a traitor, but morally he's not. What he's doing is good for Scotland and it is certainly right from his personal point of view given what Macbeth has done to his family.
This question has a number of different dimensions to it. Firstly, I think it is clear that if we examine Macduff's motives for leaving Scotland and joining the forces of Malcolm in England, we can see that although Macbeth would have undoubtedly called him a traitor, Macduff's actions were for the good of Scotland, his country that he loves. Note what he says to Malcolm in Act IV scene 3:
O nation miserable!
With an untitled tyrant bloody-scepter'd,
When shalt thou see thy wholesome days again...
Macduff's actions in fleeing Scotland are done for the ultimate good of Scotland, which he sees as being ruled by a usurping tyrant: Macbeth. Thus I don't think we can argue he is a traitor from this perspective.
Where there is a failing in Macduff is in the way that he left his wife and family to be slaughtered by Macbeth. Yet we can't accuse him of being a traitor because of this. I honestly believe that Macduff never dreamed for one moment that Macbeth would sink so low as to kill his wife and children. His failing is in his underestimation of the extent of Macbeth's evil, rather than treachery. Thus overall, I don't believe that Macduff can be called a traitor.