They were, many character ideas 1st appeared in the strips before appearing in the comic books.
Sort of makes you wonder why no one credits these strips for the origins of these characters. So far off the top of my head the strips have the first bald Lex Luthor, the first comic telephone booth change, Mxyzptlk, Bizarro, Metallo, and a proto-Brainiac. What major characters (separate from say the first couple years at Superman's inception) are left? I guess Superboy and Supergirl are the obvious ones, but what about classic villains?