This was really relaxing to play and the grass looks amazing! Your 3d graphics in this are fantastic, just like your other games :)
I think some small feedback would be that the first three levels seemed to require nearly all the grass to be mowed and I had to search around a bit on level 3 to find the small bit I missed. A slightly lower threshold to allow for some missed grass would have been nice I think.
Otherwise the only thing I can think of is just adding some nice occasional background elements like a bird swooping in to peck at a fence and flying away if you get too close. Or maybe just getting to mow the lawn in different seasons so you can hear the crunch of snow or fall leaves. I think adding in more gameplay elements would probably detract from the relaxing aspect of the game which I really enjoyed.