It could be the ground is smooth but the grass is not. Grasses are two types, spreading and clumping. With the spreading types, holes in the grass "canopy", that naturally develop, usually get filled in over the winter or next spring as the roots spread. One plant fills in around it and spreads to make more plants. Think fescues.
There are also clumping grass varieties like tall turf fescue that are one seed means one plant and no spreading. If the grass dies out around it, it won't fill in. The lawn will slowly fill in if blended with some spreading fescues. Also, tall turf fescues are hard to start when planted and need extra time in the fall to develop and survive the winter.
Last year in Ohio was a fall drought when your seed was likely planted and winter was tough so if tall turf seed did not get an early start (pre Sept 10th) and a pleasant fall (it didn't), it was damaged over the winter. Check your seed and see if it's tall turf fescue and advise. If it is, and since tall turf fescue is the rage these days, my guess is the seed was tall turf fescue, got a late start from the drought and then the cold winter damaged some of the grass that did start. The lawn isn't established enough to rebound this fast so there are holes and bumps when you mow.
For my own lawn, the dirt is smooth as can be but this year I have lots of holes in the grass cover from the dry summer/fall and mowing is as rough as you describe. I just grin and bear it till it recovers.
Edit--Grass benefits from fertilizer in the fall after things are dormant. In Ohio, that's just before it snows and in the early to mid November frame. Grass roots develop over winter and in early spring. A high nitrogen one like 32-4-10 works well at 250 lbs per acre if you are experienced and 150/acre if not.. No spring fertilizer until around May when the initial burst of growth is over. That's something like a 21-2-10 or similar. A smooth lawn is dependent on fertilizer and fall is most important.
Here are pics from my yard today showing the pockets of no grass. Ground is flat as can be but grass isn't.