Basically what other people said, he received training from King Kai, Yamcha was stronger than Krillin before Krillin got his potential unlocked by the elder, he was stronger than Krillin when they fought Vegeta and Nappa( at least according to official power levels).
He just became a gag(or always was) being the guy that always ends up with the short end of the stick and beat up, then he became a baseball player and forgot about his training, even if he is rusty right now I doubt he is weaker than Roshi.
P.S: Wow seeing your other posts seems like you just hate the character, I don't care that much I'm more bothered(or find it stupid like I said above) that they took the old man with them, not that Yamcha wasn't in Freeza's arc or will be in the tournament.
I actually don't hate Yamcha and actually believe he got a raw deal in the manga due to Toriyama
introducing Krillin and just plain sucking at developing characters (for the most part) it's just fun to bust his balls.