Yes it's a point I've pondered myself. Living amongst them tho I see they may be stupid (your term) but they have their own personalities. I've met many people who are just as stupid (you know it) as sheep but they can still be worthwhile and lovable. Intelligence is not a measure of eatability or you'd be eating all the stupid people. Yeh the lambs have a short but happy life gambling around but it is less than a year. They are huge because they are supplemented before being ready to kill, to 'fatten them up'.
I see them playing in the fields next to mine and then suddenly they are all gone and the poor breeders cry for weeks. (The cows are even worse, they keep me up at night with their howling). Then the poor sods are raped, they even call it a rape rack, and pregnant again. They give birth twice a year and their babies are taken twice a year. This goes on until they can no longer push babies out. They constantly need help. Like cows they are spent by age 5 and useless to the farmer so sent to slaughter too for pennies on the pound. Yes I believe they did the same with human slaves. Bred them.
So do I think happy slavery is ok? No. Obviously.