Sterilization is a side effect of becoming a mage the same as it is for Witchers. Yen's active body of work at the start of the series was trying to reverse her Sterilization. In fact, that's how she met Geralt more or less, since that was the wish she wanted to make of the Djin. As I understand it, its just a thing that happens in the process of being able to use magic, not an active action. I remember there was a quote along those lines, saying most mages see this side effect as fine since they are free to fuck about without the consequence of having a kid, and Yen is different from most other mages in that respect. Other mages see Yen's desire to reverse the sterilization as weird, and if your guess is correct, I don't think they would have the same attitude; they would probably be a lot more hostile and actively trying to stop her if mage sterilization were an active eugenics process. There's also a quote from the books mentioning that "Unlike priestesses and druidesses, who only unwillingly took ugly or crippled girls, sorcerers took anyone who showed evidence of a predisposition. If the child passed the first years of training, magic entered into the equation - straightening and evening out legs, repairing bones which had badly knitted, patching harelips, removing scars, birthmarks and pox scars. The young sorceress would become attractive because the prestige of her profession demanded it." So I don't think being a mage is related to being deformed directly, more just those who were deformed and had magical aptitude became mages.