Trade war or not. This particular move makes sense though. There is no reason to continue giving China - an economic superpower - a discount on mail.
Yes, working to negotiate a change to that particular element of the UPU price structure makes sense (which is why Canada already did it for China-Canada mail.)
Whether rage-quitting the UPU makes a lot of sense (even as a negotiating tactic), is a different question; the UPU is more than a Chinese discount.