I always wondered what it did can you help
Nice is just like congratulating a person. So if there shout out was something along the lines of "Shiny #6!!!" Then some people would got the nice button. Others have used it to simulate in saying gg of sort. On a side note on your birthday "Nice" turns into "Congrats!" and you get a small birthday cake next to your player.
Nothing more than just saying "nice" to someone, either after a trade, them giving you an o-power, or just randomly saying it to them because you're feeling nice.
Or as Oncoming $torm said, can be used as a sort of "gg" after a match.