I don't know what the current draw of a typical lawnmower is, but 12A @ 110v RMS = 1320W; your inverter won't be a problem (an APC UPS would be ideal, in fact), but the batteries are going to weigh a ton! I have a 3000W UPS here (and it's capable of delivering that, for about 10 mins, from fully charged), and I can barely lift the thing. I'd certainly hate to have to carry it around while mowing the lawn; and if it takes more than 10 mins to do your lawn?
On the other hand, battery lawnmowers have existed for years, I'm sure if you took the old lead acid battery off & replaced it with modern LiPo batteries, you'd get better range & power for less weight.
But surely, the mad-modderist of all, would be to get a quadrocopter, fit blades underneath each propeller, and then fly it over your lawn at the correct height?
