Wu Sangui was a Ming general. He defected from the Ming and joined the Qing, allowing the Qing army free passage into undefended Ming territory. That makes him a traitor, whatever way you look at it.
History agrees on this, because the reason his rebellion ultimately failed was that he was widely known as a traitor in the eyes of Han people, and did not receive widespread support from the common populace.
You can try to rationalize his actions, but he is still a traitor by the very definition of that word.
As for Chen Yuanyuan, it's difficult to sort out how much is history, and how much is just popular legend. Are there any historical texts that say Wu defected to the Qing because of Chen Yuanyuan? Maybe that part is just embellishment and storyteller's romance.
Finally, considering that Wu killed one of the last Ming emperors, it is extremely unlikely that the Zheng family would've allied themselves with him. Don't forget that Wu wanted to be emperor himself, while Zheng wanted to restore the Ming emperor to his throne. The two had directly opposing goals.