Well, to some degree it might be possible to selectively breed plants to prioritize root production a little more. I’m not sure to what extent that’s feasible though. You also have to understand, plants have evolved to actively want to be as big as possible. If you’re a small plant, your neighbor is more likely to grow larger than you, which blocks the sun from getting to you, which will cause you to die. So trying to make plants smaller in and of itself has ecological risks. Or at the very least, such naturally short lawns would be a lot more susceptible to weed encroachment.