China is on a mission impossible. To be a good mediator, one has to be trusted by both sides and has the power/influence to drive the agenda.
China has claimed publicly that it is the unlimited partner of Russia. In other words, it supports Russia's invasion. So far, it has not condemned the invasion. How can the Ukrainians trust Russia's partner in crime to be a good-faith mediator?
Many years ago, China organized so-called six-party talks on North Korea de-nuclearization. That marathon lasted many years, only to see North Korea turned its nuclear ambition into a reality. What a joke!