Wear items aren't generally covered under warranty. Seats and carpets are wear items.
Though I'm not sure you're asking about seat covers, an additional cover could help, but keep in mind that the vinyl on the bolster gets stretched when you sit on it and stretch it, and if the vinyl doesn't have enough elasticity, it will crack. That may not be solved by a seat cover, as it will continue to stretch underneath the cover if you sit on it the same way. Vinyl treatments that enable the material to retain some elasticity will be necessary to minimize further cracking regardless of using a seat cover.
Additionally, the more often you can get into the seat with as little pressure on the bolster as possible, the less it will continue to crack. I guess that's pretty obvious though.
This all said, it sucks that a 2-3 year old vehicle has seats that are cracking.
I use leather conditioner to keep the V-Tex (vinyl) from drying out. It is also a good idea to use a sun shield in your windshield, especially when parked during the summer months. Native vinyl is a rigid plastic, but gets modified with plasticizers (to make it soft and pliable) and UV blockers to provide protection from the sun. However, everything has its limits.