A lot of cheaper lawnmowers don't come with a padded handlebar grips, and the ones that do can start cracking off over time. You should do yourself a favour and upgrade it with handlebar tape.
This tip, courtesy of Allan Watkins, is really obvious in hindsight, but it's the kind of thing you may never think to do. He used bike handlebar tape, but tennis racket grip would probably work really well too.
It won't help you mow the lawn any faster, but it should at least guard against painful blisters.