I have an electric (though not robotic) lawn mower, and it turns out that it's not much quieter than a gas powered one. No engine noise obviously, but the blades spinning and hitting grass still makes a lot of noise (and indeed in my case it turned out to be the vast majority of the noise). So it wouldn't be a very good idea to run your robot lawn mower at night.
Maybe your specific model? I have a Makita electric mower and the difference is night and day. It's very relaxing and I mow with no hearing protection. I've tested to see how far the noise carries and none of my neighbors can even hear it. Meanwhile, I hear gas mowers from my neighbors on all sides and several lots away from me.
