Made by caL (or calebci on Reddit), it's exactly what it sounds like: players shouting nice at one another, in varying forms, as they win rounds. "Nice." "Nice one." "Nice boys."
There's not really much more to it than that. Calebci has said on Reddit that they will be making another one for further stages in this tournament, so in case three minutes and 31 seconds, and over 100 utterances of NICE, isn't enough, there's more to come.
Perhaps you can think of a hot take - that it shows a lack of creativity. Or, the much better take, that this is just a sign of how all of these players, who often speak completely different languages, can all communicate with one phrase.