After they make the jump to hyperspace and the First Order fleet catches up they quickly get out of range of the FO gun's. Since we don't know the effective range of the dreadnought, there are two options:
Assuming the dreadnought has a similar effective range, it would never have been utilized again. The fleet would have stayed out of range while in space, and then there was an energy shield protecting the base at Crait. So it's useless in both cases.
If the dreadnought had a further effective range then the rest of the first fleet, then it could have been utilized to bombard them in the early part of the 'standoff'. Though throughout the film the gap continually grows, so eventually they would have been out of range of the dreadnought as well. But we can't say how much damage would have already been done. But the large cannons on the dreadnought appear to take some time to charge after dropping out of hyperspace, so the Resistance would have a good head start. Especially if the dreadnought doesn't have the energy to both charge the guns, and accelerate much.
It's also worth noting, that even if the dreadnought had a further effective range, that the Resistance could have launched a bombing run at that point as well. Though the First Order may have been better prepared, it would have similar odds of success.
So in all likelihood Poe's actions did not help the Resistance in this case.