Panel 1: It would appear that Mike was getting ready to go bowling today. Unfortunately for him, Elly has shot that down. When Mike asks why he cannot, Elly tells him that he knows perfectly well why he can't.
Panel 2: You see, "we'd" "agreed" that he couldn't go bowling if his room weren't tidied to the insanely high standards Elly has. Mike yells "OH, COME ON!!!" at the back of her head.
Panel 3: When frustration leads to a violent outburst because he isn't really allowed to win arguments and thus doesn't know how to handle conflict, he's criticized for being too old for that sort of thing. What he needs to do is act like an adult and get it done.
Panel 4: Given that the only adults he knows appear to hate the happiness they can no longer feel and hate kids for being able to enjoy life, he thought-bubbles that "adult" is "dull" with an 'a' and 't' stuck to it.
Summary: It's always sort of depressed me that John and Elly are such bad salespeople for being adults. Most of why Mike fears adulthood is the fear of becoming a whizzled-up old fart like John and Elly appear to be. They deserve their fate of being bad examples for thinking that if they were seen to smile and laugh, the children would stop taking them seriously and walk all over them. They also wouldn't respond well to reminders that they're so over the top grumpy, the kids don't respect them and regard them as people to do an end run around. That being said, they've made the children such poor fits for the real world by their shitty parenting, the dumb bastards become dependent on them as adults.