No he wasn't incompetent. He was actually a fairly competent politician (btw, he was not a military general as some people seem to think).

I think he owed his position a little to the eunuchs, and to the fact that he had a pretty half-sister, the Lady He, who became wife of Emperor Ling. When Lady He entered the palace, He Jin was made a government official. He subsequently rose up the rankings as Grand Administrator of Yingchuan commandery, Palace Attendant, Court Architect and Intendant of Henan commandery.

In the year 184, he discovered the plot of Ma Yuanyi (for a rising of Yellow Turban rebels) and he was made Marquis of Shen. Later he was also made General-in-Chief, commanding the Five Regiments of the Northern Army for the defense of the capital city Luoyang. He succesfully purged the capital of rebels and traitors and sent his Major Fan Zeng to deal with Yellow Turbans in Nanyang commandery.

Following the Yellow Turban Rebellion he came in conflict with the Regular Palace Attendants. I think this is where most people get their "He Jin is incompetent" opinion from: they say he was indecisive and made a dumb decision by calling in warlords, particularly Dong Zhuo.

His indecisiveness actually helped him: because one of his opponents, the eunuch Jian Shi, had plotted against him, tried to kill him, but the plot of failed and proof of it fell in He Jin's hands. Subsequently He Jin had him executed. A free kill for He Jin. Ultimately he was beheaded by the eunuch Qu Mu, who belonged to a eunuch faction which seemed to have been led by Zhang Rang, He Jin's other main eunuch rival. This was a very sudden, not planned, surprise attack by the eunuchs. Not much He Jin could've done to prevent it. Following his death his allies stormed the palace and slaughtered all eunuchs. Dong Zhuo later entered the capital and started his reign of tyranny.

My opinion on him is that he suffers a lot from Romance and Koeis depiction of him. He climbed up the rankings rapidly (perhaps owing his first title to Lady He, but not the rest imo), and if he was so incompetent he would never have climbed up all the way to become General-in-Chief. He was also very popular among his fellows, but I think if he truly was incompetent he would not have been. There would've been jealousy or animosity towards him, a man of low background, climbing up the way he did.

by the way He Jin has his own official biography in the official history work of the Later Han History of the Later Han.

It has been translated into English at Gongjin's Campaign Memorials: http://threekingdoms.wikia.com/wiki/He_Jin_%E4%BD%95%E9%80%B2#History_of_the_Later_Han_official_biography