Apart from the fact that it was a Bolton screwjob, would sacking Duskendale have been a sound move strategically for Robb?

No. At a time when Robb was trying to strategically retrench by retaking the North and trying to hold the line in the Riverlands, an offensive action that risks valuable manpower would require some major upside to justify it. But Duskendale doesn’t have any upside – it’s not a vital strongpoint in the theater, it doesn’t allow them to threaten King’s Landing, it doesn’t allow them to degrade the effectiveness of the Tyrell/Lannister army, etc.