I think this has been happening for about a day. As you can see in the video below, when I try to play Mario Kart 7 on Pretendo, I can’t get matched up no matter how long I wait. At first I thought it was because of the time of day, but this happens at any time of day, and I tried switching servers with Nimbus, but it didn’t solve the problem. I thought maybe my 3DS was broken, so I tried it on a friend’s 3DS, but the same thing happened.
What kind of phenomenon is causing this?
Check your NAT type
If your NAT type is fine, there might just be nobody online atm
I’ll do the same, 'cuz i’ve been having this issue since 2-3 days ago.
I checked my NAT type connection. I’ve got the B type, Although it is not as closed as it would be to not let me connect to anything, it still makes my 3ds refuse to match with other people on MK7 
Sometimes it wants to do it, sometimes it doesn’t.
Also pay attention to when the cancel button appears. If it pops up instantly try again later
What version is your game? You need to be on the latest game version to matchmake (as nearly everyone else is also on latest version).
To Update:
Technically, you actually don’t (yet). That said, if for some reason you aren’t on Mario Kart 7 1.2, you are at risk
You do if you want to match with other people on later versions. And the vast majority of people are on latest version, so will be very unlikely to even find a match.
The version of Mario Kart 7 I’m using is 1.2.
I think the issue of not being able to match started around yesterday.
It says that on the Title Screen where it says “Online Multiplayer”?
In my case I have everything up to date. Though I’m still getting an error after minutes of finding people…
UPDATE: I managed to DMZ my 3DS so that it allowed my console to connect through a NAT A type connection (code 110-00). However, it still refuses to find people for minutes until a “connection error” pops up.
@Melon3ds are you still having issues?
No, you can currently play online without any problems.