Junkrat's character is loud, gross looking and really not someone you'd ever want to meet, but somehow his personality never pisses you off to the point that you just go "screw this guy". What makes Junkrat Likable is Junkrat is you when you are having so much fun you don't care what everyone else around you thinks about you. Junkrat's entire character is a guy who's entire life is having so much fun he couldn't care about anything else, I mean the man barely wears clothes. That's what we connect with in Junkrat and THAT'S why he's likable. We remember the times we got into a situation where we may have done something stupid and later regretted it, but at the time we were having the time of our lives.
Even his ultimate feels like someone is just raining on your parade when it gets shot down. He's also not really a hero you may play tactically like Soldier76 or Widowmaker, he's the kind like Winston where you jump into the fray and just see what will happen.
JUNKRAT IS LIKE ME BUT WITH A GRENADE LAUNCHER!!!