Here you will a collection of links to all sorts of Mario Maker related places.
The Mario Maker Discord is probably the biggest community Discord server. This is also the place where a bunch of the races are organized from.
Black Dragon’s Kaizo Mario Maker Wikia
Black Dragon’s Kaizo Mario Maker Wikia page has a huge collection of Mario Maker kaizo tricks, complete with animated gifs as examples.
One Year of Mario Maker (and beyond)
Home of Kiavik’s One Year of Mario Maker (1YMM) project, and of course the sequels as well. They also do a bunch more community projects that all consist of a high number of quality levels.
The Super Mario Maker pages on Reddit. There’s usually plenty of discussion and level sharing happening there.
They also have a Discord server.
If you’ve ever watched a viewer level stream on Twitch, they were probably using Jaku’s Warp World as their automated queue system.
[SMM1] Nintendo’s Super Mario Maker Bookmark Site
On Nintendo’s Super Mario Maker Bookmark site you can easily check out level previews and stats, and even add levels to your in-game bookmarks if you log in with your Nintendo id.
Hopefully one day Nintendo will create a bookmark site for Super Mario Maker 2 as well. Until that time we’re stuck with the in-game system.