Electric is definitely about as bad as gas for mowing a lawn. The gas you're not using is just being used by someone else, if not more, because you're helping subsidize the petrodollar economy by paying orders of magnitude higher cost for batteries+motors+solar power than you would pay for a push mower or probably even a gas-powered mower. But you also have to factor in the other environmental damage from manufacturing and shipping the batteries and solar panels which you make it really difficult to avoid by demanding the mower be electrified, whereas if the mower was push-powered it would be up to suppliers whether they feel like wasting their fuel shipping it across the planet or just manufacturing it with materials found closer to you