Weight reduction certainly does have an impact on mpg, but saving a few pounds on a windshield is not something normally done. In fact the trend is to make them heavier with sound deadening in mind. A few pounds has an unnoticeable impact on mpg, but can have an outsize impact on the all important EPA rating. The EPA has weight brackets, get one pound below a particular bracket and the mpg rating jumps. So when a car like a hybrid really needs good numbers for marketing, manufactures often take some shortcuts. The base model Niro is a good example as they did a lot of dumb things to lighten the car to move the bracket. Like the flimsy load floor in the base model, or the lighter dash materials.
While anecdotal mpg figures have a lot of variables like driving style, my LX with tech has just finished a full four season year with a measured average of 52 mpg. That is better than the base model. Disclaimer, I did lighten the car and removed the non-aero roof rails, but it still weighs more than the base model. Not sure of how much as I haven't weighed the car, but at least 50 pounds more is my guess. I do have a smooth driving style and minimize my speeding up and slowing down (about 75% cruise control) so weight differences on efficiency are minimized.
2018 Kia LX HEV Metal Stream with Advanced Tech