From article silverwook posted:

"The one-hour new series premiere was the #1 program from 9-10 p.m. among all major kids networks in kids 6-11, kids 2-11, tweens 9-14, teens 12-7 and persons 2+"

I am really not surprised. The "movie" did bad because parents didn't want to dish out ten dollars for a ticket. And so many people were unsure about it. The TV show was free, so anyone could feed their curiousity for free.

Not surprised it is the highest in Cartoon Network history, as it is kind of a niche station, Star Wars is a popular franchise, so I am sure it has pulled a lot of non Cartoon Network viewers in.