You have to understand Aqua's mentality, she's built her entire self-image around being good at video games. She's low IQ, autistic, friendless outside of hololive, romantically hopeless and unable to function by herself. Despite all of that, she's good at video games, good enough to make a career out of it and be known as the best gamer in hololive. Gaming success was her source of self-esteem when the outside world was too harsh and scary. Yet when everyone was watching, in a company-wide event, she lost. Worse than that, she lost to Suisei, who is basically Aqua's opposite: attractive, self-assured, intelligent, extroverted and multitalented.
For Suisei, a Mario Kart tournament is just another victory to be proud of, but for Aqua, she just lost the only thing that defines her as a person. If Aqua isn't good at video games, what's left?
