For a number of reasons, this action from the young fan can be deemed extremely rude and improper.

Especially for a superstar athlete, asking for a picture and then disparagingly criticizing the player’s team is blatantly disrespectful and bad sportsmanship.Regardless of the outcome of the game, the young fan should be expressing more respect and gratitude for the athletes. It comes across as haughty and unsportsmanlike to make big guesses about his team defeating the opponent.As a well-known player, Doncic probably expects younger fans to show some respect and admiration. This child clearly deviated from that expectation in behavior.This kind of contact presents a negative picture of the young fan and provides a poor model for behavior among supporters, particularly the younger ones, toward professional players. Even if their team loses badly, supporters should keep a degree of respect and humility. 
Although some trash talk or pleasant banter among fans is typical, given the young age of the fan engaged, this seems to transcend a line into pure disrespect. It’s a wonderful reminder that athletes—even superstars—deserve to be treated with decency and ordinary courtesy.