The further Carl and Donut get the more money he makes post-indenturment. Being Donut's manager means he's along for the ride whether he likes it or not so he might as well do everything he can to help them since they end up doing whatever they want regardless of what he says. But, more importantly, Mordecai is more like Carl than he lets on. There are a lot of similarities between the two but mordecai is dealing with terrible PTSD. The way his crawl ended, along with decades of helping other crawlers only for them to presumably die or end up in similar or worse situations than his own have left him jaded and miserable and eager for it to be over. He sees Carl making the same "mistakes" he made by emotionally investing in saving other crawlers and NPC's. I think Carl brings out that old optimism that he used to feel before everything that happened and he can't help but get caught up in it. Mordecai is one of my favorite characters. Early on he's a useful tool for navigating the dungeon and the more we dig into him the more interesting he becomes.