Fat tastes better than lean meat. I think it would be desirable to make livestock fat. Historical records indicate people like fat animals for food. Biblical texts refer to flattened animals as desirable.
Up to a point. Producers have to meet a standard for supermarkets (at least in the UK). They get financially penalised if animals are too fat when slaughtered.
Maybe for ground meat, but beef for example is priced higher according to it's fat content. For example prime is higher in fat and in price than choice, and wagyu is prized for it's high fat content. The fattiest red meat, A5 wagyu, is upwards of $100/lb.