I put a Mending Moss on my Hattock (Certus Quartz/Living Wood/Living Wood) and noticed, that it does not work.

I think the xp buffer for the level ups replaces the one for the mending moss maybe?!

XP drops have absolutely nothing to do with tool leveling, and TiC adds neither certus quartz nor living wood. You need to hold the mattock in the hand while picking up xp.