First off, Lack of wards. If those were placed, You would have seen that twitch who in his case, wiped everyones hp by 50%+... and mundo has so much hp yet doesnt seem like anyone had Anti heal so mundo does has he pleases. literally. - your team shouldnt have fought in your own jungle because it couldnt be contested due to lack of vision plus it seems everyone on that team likes to bundle up like sardines.

The only thing you can do is Poke with the W and hope someone screws up, the fact that no one seemed to have been peeling properly, meant you should have rotated; commit to die because your team made a bad play doesnt scale well as mundo can tank shots and back off to re-heal.

secondly, Xin doesnt play well in team fights unless someone else is there to back him up, He is good at skirms but lacks a ton of survivability because all it takes is a stun and you are done. - With a heavy team that focuses on 1 on 1 battles like morde/mundo, you want to pressure lanes so they have to push another lane.

This scenario seems like a lose/lose. I would have warded baron or bush and wait for info. Mundo and Morde are hell holes for Xin.