Mario Maker does have online.
There is a mode where you can connect to the internet and download other courses.
I don't know if you could upload courses you made on the 3DS version or not. Many people say no.
We'll find out on December 2nd.
And that's where you're wrong, kiddo. Despite there already being servers with countless courses for the WiiU version, they decided to go all-offline for 3DS. You can only exchange courses via StreetPass, which is a complete nonsense if you're not from Japan where every person has six 3DS consoles.