Do you think China want to help us? or is it more likely they want the USA to remain mired in this swamp for another 20 years?

Even if they don't, you gain some diplomatic points by putting a spotlight on their recalcitrance. If nothing else, if and when you end up having to take more direct action, it's one less thing for other nations to second-guess you about. At least you asked, and China said no - if you hadn't even asked, people could after-the-fact claim that you didn't exhaust your options for non-military solutions to the problem. Which is probably why this ask is being made (or repeated, most likely) in public rather than just diplomatic channels to Beijing.

I mean, this seems like basic Diplomacy 101. It would be malpractice to not even make a public ask of China to talk to Tehran - it's all upside, and it's a critical step even if you know they're going to refuse to get involved. There's no incompetence on display here; it would be a sign of incompetence if an Administration's State Department didn't check that box as part of their efforts.