Can't help but I find it normal for Mercy to call him as Dad. At first Blizzard labeled her age as 34 year old but in Winston's files in Recall she was shown to be 37. Value on the site has since been changed to match the one from Recall. So no wonder she would see him as her role model, as a senpai to look after even though she is still in her mid-thirties. After all she is still young compared to Jack who is more than 60 (?!) years old?