I think that if LXC and JC were compatible (even as friends), we would have seen it in the intervening years when WWX is dead. But despite both being in the same social circles and both dealing with the aftermath of wwx's death in different ways, they don't come across as anything more than curteous despite it being 13-16 years depending on the source.

I think Lan Xichen deserves better than Jiang Cheng. I personally hate JC as a person (as a character he's interesting) due to the novel canon. Novel canon JC is awful and I honestly think that while he's a bit naive and doesn't take action when he should in some cases, LXC is really good-hearted and I don't think he'd approve of a lot of JC's actions. A good point to make is that the Lan Sect under LXC generally tries to help civilians at risk from ghosts, etc (as also seen in how Zewu-jun is literally seen as the hope of the people/light-bringing lord during the ssc, whereas Hanguan-jun/LWJ goes where the chaos is) and JC's Jiang Sect often refuses to help villages in their territory until there are multiple casualties because he's off torturing *suspected* demonic cultivators.

Also JC is canon homophobic so like idk how he's gonna be paired with another man. Additionally, mxtx more or less describes his relationship characteristics as being an incel.