There are several possible reasons for bumpy lawns. Sometimes the repeated freezing and thawing conditions of winter and early spring move the soil up and down. More established lawns may become rough and uneven over time as the turf grass gradually thins out. Thinning lawns can be caused by shade, insect damage, and poor maintenance practices. Re-establishing a healthy, thick turf will help improve this situation. Another possible cause of bumpy and rough lawns is the presence of earthworms. If night crawlers are causing your problems then you will notice small, cone-shaped bumps randomly distributed through the lawn. Night crawlers leave behind a waste product called castings that deposit at their burrow entrances forming cone-shaped mounds at the soil surface. Run a core aerifier over the lawn several times in different directions to break up these cone shaped mounds and compacted clay soil as much as possible. Allow the cores of soil to dry partially before raking them back into the lawn. This best done in late summer, mid-August through September. If you do it in spring wait until the ground is firm underfoot and before hot weather sets in. Apply a pre-emergent herbicide to prevent annual weeds from sprouting. Core aeration will also loosen up compacted clay soil, and thick layers of thatch. A thatch layer will develop if organic matter is produced faster than it is decomposed. Another way to improve bumpy clay soils is to top dress with ½ inches of Leaf mold, compost or peat moss. Deep depressions may need to be filled in with soil and  then reseed over the filled areas. The following sites have more details.
https://extension.umn.edu/lawncare/how-control-thatch-your-lawn
https://extension.umn.edu/yard-and-garden-insects/nightcrawlers

https://hort.extension.wisc.edu/articles/lawn-aeration-and-topdressing/