Yes, an e-vignette is mandatory for all rental cars, motorhomes and campers.
Usually, Slovak rentals do have a valid vignette arranged by the rental company, foreign renting companies most likely do not.
In order to avoid a heavy fine we suggest to make sure with the rental company upfront that the car has a valid e-vignette.
Related articles:
What vehicles need a Slovak e-vignette?
Where can I buy the Slovak motorway e-vignette?
What happens if I drive without a Slovak e-vignette?
Which roads require a Slovak e-vignette?
Can I drive into Slovakia without a valid e-vignette if I avoid motorways?