Crono isn't just some random kid that showed up though. He was predestined to save the world, predisposed with natural fighting talent to build upon during his adventure, and bravery to face whatever challenges might come his way.
Frog might as well be a knight, but he technically isn't. He is the legendary hero of 600AD, also had some natural talent which he had already honed by the time of the adventure, but he doesn't seem as brave. Now don't get me wrong, I'm not saying that he's a sissy. He admitted himself that he wouldn't be good as a knight because he didn't know if, when the time came, he would do well on the battlefield.
I guess when you compare the two, it all depends on the time during the adventure that they face off. If they bash heads near the beginning of the game, Frog would probably win due to his higher experience with the blade. By the time they face Magus, it would probably be a draw or a very close match. Crono would've honed his skills by facing all of the monsters during his quest while Frog would've just had gotten himself back up after sulking for awhile. They would therefore be pretty evenly matched. However, if they faced off at the end of the game, Crono would most likely win. You have to agree that he had to have gained some mad skill to be able to defeat Lavos. I know that the others helped and that he couldn't have done it without them, but Crono is given the credit probably due to the fact that he had become a great swordsman. Crono would be the hero who saved all of time whereas Frog only saved one time period. It would be a close battle, but Crono would win during an encounter at the end of the game.