There is some truth to the hybrid battery weight making the vehicle ride better. I would describe it as the vehicle feels like it is "planted" or more sure-footed on the road than the same vehicle in an ICE version.
As to needing the Limited trim to get the heated seats and heated steering wheel (bliss!), yes, this can sometimes be the way car makers package their trims. One thing I've noticed in my search for Lexus RX350h and Toyota Crown is that when you move up from the base model, because of the way they package optional equipment, you might get bigger wheels and tires as part of the package.
For example, the Toyota Crown sedan, if you want the 3-D multi-camera parking system you must move up to at least the Crown Limited. Also, if you get the 3-D parking system, as part of the package, you get 21" wheels, up from 19" wheels. Larger wheels typically mean low profile tires (less rubber height in the tire) which means slightly worse road manners when going over bumps. Not a deal breaker, but there is a detectable difference in ride quality. (And replacement costs when you need new tires.)
I'd check to see if the Highlander Hybrid Limited moves you up to a larger wheel/tire size. If "Mama" wants heated seats and steering wheel (bliss!) she might need to take larger wheels/tires to get it. Oh well...