I know there used to be one that wasn't very popular that was published by one of the Korean publishers like Aeria or something but it died due to small population. Your best bet for something like that would probably be playing the actual mario kart games online if you have the ability, or The Crew would be the second closest thing even though it's more of a realistic racer.
The Crew might not be cute, but the racing style is a lot closer to Mario Kart than a sim game. If you can borrow it from a friend on a console, give it a try.
Just wondering if anyone can suggest a kart riding MMO with power-ups, customizable character and kart? 
Comments
Executive Editor (Games) http://www.wccftech.com
There used to be Kart Racers Online and Come On Baby which was really fun but both are gone.
Your best bet is playing the real Mario Kart in online mode.
Thank you everyone for your replies! (And yes I know Mario Kart isn't an MMO, I was looking for an MMO like it).
The crew is a car racing game, I'm looking for something on the "cuter" side. It's a shame all the similar ones were shut down
Oh well! Thank you.
xoxo
The Crew might not be cute, but the racing style is a lot closer to Mario Kart than a sim game. If you can borrow it from a friend on a console, give it a try.