Background on the joke: Before becoming Solider 76, he was team leader of Overwatch. As such, a running joke is that he's 'Team Dad'. It helps that a lot of his in game voice lines are stuff like 'back in my day we'd have this payload delivered by now' and 'Get these jokers off my point!'. Cementing him as the crotchety old fart. D.Va on the other hand is the youngest hero. She's a professional gamer turned mecha jockey and is loud, arrogant and seen as 'team baby/daughter' to a lot of the other heroes.
Bonus point: The guy in the skull mask is Reaper, who was 76's best friend before they got into a fight and blew up their headquarters. He was leader of the sister team Blackwatch, which did all the dirty jobs. Fans depict him as a 60 year old Edgelord and he and 76 are often depicted as co-dads to the team, or rival fathers living out their feud through their kids.
I know, if you have to explain the joke.. But unless you are into the fan community around these characters, you just wouldn't get it anyway.