As far as I know it’s never been done but why not. I’m sure a character like “Tony The Tiger” could carry his own show. Think of how much the sales would go up for the cereal companies if they had a 30 minute Saturday Morning Cartoons on TV.
I’ve already thought about this and I really think it would work.
Didn’t Sugar Bear have a Saturday morning cartoon?
Yup, it’s been done. (You didn’t really think that there were any new ideas, did you?) 
It was called Linus the Lionhearted. Just like blowero remembers, one of the segments starred Sugar Bear. Linus was the mascot for “Crispy Critters” cereal, SoHi was a little Asian boy who represented Rice Krinkles (Post’s version of Rice Krispies), and a postman named Lovable Truly was the mascot for Alpha-Bits.
The web site above has the theme music.