If I know what you're referring to, the motion blur is the stars, not the comet. That's because Hubble is tracking (pointing at) the comet, not the stars. The comet is therefore not blurred in its direction of travel, while the stars appear to be moving in the direction opposite of the comet's travel. To the extent that the comet appears blurred, that's presumably its coma.

At ~100s, it's already at about the minimum for Hubble; often it's 1-2 orders of magnitude longer.