Asuma was already a strong jounin squad leader, proficient in fire and wind release techniques, he was able to use chakra flow to his trench knives, and was able to become one of the 12 ninja guardians.
basically he was average powerful for a jounin.
Since he was the son of the Third Hokage, shouldn't he have been a little stronger. For Example, look at the Sanin, who the Third trained, were very powerful and are feared, so how is that Asuma was just average??