No they're not an effective means to reduce car traffic
In my country, these scooters are geo-fenced to the inner city, they simply stop working if you go to the outer districts, which kind of negates their intended/advertised purpose. My place of work is well within city limits, yet outside the geo-fence. Thus, I would not be able to use one.
The geo-fencing makes sense for the scooter rental company as it limits the service area. After all, some poor, underpaid souls need to collect/replace them using gasoline-powered vans each night. So much for clean, electically-powered mobility. From a potential user's perspective, the geo-fencing precludes meaningful use, i.e. inner-city commute.
I mostly observe some kids driving them, themselves alone or with a passenger, for fun. If the scooters reduce any means of traffic, they reduce short walks, e.g. from the flat to the bus stop.
What I could also observe over the last years, since they popped up in my city: rental scooters make people behave anti-socially. Scooters are left on the pavement at the very spot the driver steps off them. Hence, they block parts of the pavement next to the entrances of appartment buildings, at bus or tram stops. People drive them on the pavement with all the associated risks of relatively large relative velocities, overtaking from behind and slightly randomly walking pedestrians.
I am not impressed by the contribution to society by the availability of rental electric scooters. I don't see many benefits, yet experience their downsides.