I can see the criticism of one calling themselves the GOAT
https://youtu.be/jQe0DYFBf1c?t=154
Even though Mayweather is cocky, he proves it with all those wins (50-0) and doozy plays (hugs and runs around the ring tactically to win matches via points).
Sunni Lee has beaten Biles on numerous occasions and she took the Gold this 2020 Tokyo Olympics while Biles took Bronze bringing her tally to 4 Gold, 1 Silver, 2 Bronze. She went full cocky and then failed to prove it at the big event. You can't call yourself GOAT and not live up to it. All in all she still would be considered GOAT since she holds the most medals and gold medals at the World Championships.
I think the difference here would be people calling her GOAT vs she herself claiming to be one then failing to maintain that posture. We all know Tiger Woods, Michael Jordan, Roger Federer are GOATS but they never claim it themselves (that I could recall) so even with their failures at times, it was acceptable. She on the other hand was comparing herself to the likes of Mayweather which is a false equivalence if you ask me.