This is definitely a short-sighted move by the city.
The rental ones requires plenty of vehicles to be recharged as well, which makes those technically running partially on fossil fuel.
And it's a fairly big annoyance for pedestrian most of all (i guess that's why parisian voted against those escooter rental companies).
As it is, I only compared them to other vehicles. It's not really fair to compare them to walking which is much slower and often impossible for the less than fit.
Given that animals aren't super efficient already, I wouldn't be surprised to learn that something like a moped puts less CO2 in the air to move a sedentary human compared to a human that's huffing and puffing CO2.
If they weren't allowed on sidewalks and parked in (repurposed) car spaces I'd be fine with them.