It depends on what you consider "classic". There are many beloved comic strips that never turned into a TV series for some reason or another. We just don't remember them now... perhaps because they never were turned into a TV series. I also keep thinking about comics like Pogo which were not a huge success when TV specials were made based off the comic.
I suppose you would be hard pressed to find a successful newspaper comic strip that hasn't been turned into a tv show or some kind of theatrical cartoon series in the past 80 years, there are notable exceptions, such as Calvin and Hobbes, as you mentioned. Calvin and Hobbes is no less of a classic even though it had no TV series and extremely minimal promotional items to accompany it.
Yes, but even the star of Thimble Theatre, Betty Boop, had her own TV series!
I think you may be mistaken (you or me one). I think Betty Boop came later in cartoons. I thought the original Thimble Theater stared the Oyl family. I could be wrong, but I don't think Betty Boop originated in comics.