Are there people online and available in the first place?
https://wiimmfi.de/game says yes but it doesn't mean they're looking for a public match in the same mode* as you
* don't remember if Brawl had 4 vs all/2 vs 2/1 vs 1 categories
It works like Mario Kart DS - it will wait a couple of minutes for 4 players to be found but as long as the minimum number is met it will eventually start