Yiftach wasn’t chosen because he could solve klal yisroels problems. He had a lev tov and was full of zeal for torah and klal yisroel. He was not a “gadol”, as he was unlearned.
He represented how even the lowest among the people – an outcast who was born to an unwed mother, grew up among warriors and rough men – had superb character that eclipsed the best of the goyim.