It's not selfish at all, however anyone who isn't a complete idiot will realize that it's a dodge to answering the question of whether you liked the food. You

It's not selfish at all, however anyone who isn't a complete idiot will realize that it's a dodge to answering the question of whether you liked the food. You risk an uncomfortable exchange and negative social consequences with a move like this. It's much easier to just lie and tell granny that her crappy food was good and move on to another topic. And really, saying "it was good" isn't a complete lie: it could have been worse, much worse (assuming the food didn't give you food poisoning or kill you outright), and "good" is a relative term.