What kind of mower do you have? I might be a bit iffy about them if it was petrol. I have an electric mower and my boys go straight out on the lawn and they're fine. You do have to make sure that you don't leave the grass clippings on the lawn as they shouldn't eat those.
We have an electric mower, it does an okay job of mowing because it isn't fussy leaving clumps of inedible plants.DH seems to pride himself and the Bosch in their garden skills, but always moves the run to a fresh patch and mows round it, otherwise he'd never hear the end of it. But it doesn't leave neat little pellets of fertiliser on the lawn like the Cavy Mowers.
Our lawn looks like weirdy rectangular crop circles when the Pigs are in full grass mode.
My boys have been out most of today - gorgeous day today. They were a bit miffed that I used the mechanical Guinea pig on the lawn instead of just leaving them to get the job done. Gingerpig was so sulky that he went on strike, refused to mow and instead slept for 2 hours in the sun.