If one looks at the design of the Dreadnought, or in fact most Star Wars spaceships, they lack (powerful) engines on the front. They only feature engines in the back, to fly forwards. So, for the Dreadnought to dodge, it would have to fly forwards, or fire one half of its engines (for example, to go right, fire the left half of the engines). If it had flown forwards, the bombs might hit the ship on the bridge, or center mass (which risks splitting the ship in two halves or cutting off hallways with fires or vacuums, effectively splitting the ship in two halves as well).
But, even if the Dreadnought could go in reverse, we have two other issues. If it was standing still, it has to lumber that massive surface into movement faster than the bombs that are being propelled towards if from above (where its surface is the greatest, together with the underside). In a zero gravity environment, it's really hard to get anything moving and once you do, trying to stop it is even harder. If the ship was already moving forwards, the engines would have to not only cancel the forward momentum, but then also get it moving backwards once more.
And even here on Earth (where we are not in a zero-gravity environment), generating momentum takes time. Imagine a car. It doesn't simply go backwards at full speed immediately, as I'm sure you've seen or experienced. Likewise, a car that is driving forwards, does that stop immediately? Of course, on Earth a car stops faster due to grip to the ground and resistances from for instance air. In space, you have no 'grip' onto anything, nor is there resistance (except from gravitational pull of bodies like planets). Starting to move in a direction from standstill is a case of increasing thrust on one side, like with an engine, resulting in movement into the opposite direction. Stopping works the same, only you increase propulsion against the durection you're moving towards. But that takes considerably more time as there's nothing to grip your weight to, or lose your momentum in.
So, if the Dreadnought would move forward, it risks losing more of its front part, either directly or effectively. If it could have moved backwards (i.e. it had strong engines on its front too), that would've taken too much time anyways. At least, too much time to not change much of the outcome we see in the movie.