The most obvious of these crippling flaws is the fact that he's rather short-sighted. As by way of example, he simply can't seem to quite understand that his obsessive need to make sure his mother stays home "where she belongs" might constitute anything like a problem. His eyes see a face that looks at him like he's a creep, his ears hear words that relate to how she's more than a maid for a lazy, selfish child but his tiny brain and inability to see that his needs aren't the be-all and end-all of existence prevent him from understanding why his mother wants to make his life bad by going places and doing things.
Similarly, he can't quite understand why it is that people keep telling him to not pick on the kid sister who has to be trying to destroy him and shove him aside and make him live in a dungeon because he's too self-absorbed and whiny to suck it up when he doesn't get the attention he allegedly deserves. He also doesn't quite see that his constant reminders that he personally doesn't think that she's welcome in her own home and how she doesn't deserve good things to happen to her because she gets in his way might affect her later in life.
As if this were not bad enough, the dunce who can't quite ever seem to really remember that other people actually do have feelings that matter even if he can never experience them is going to spend the better part of the Middle Years making a muddle of his love life because he can't see past his own needs. The reason the sap stood around braying about how Martha was toying with him by selfishly and cruelly reading his ultra-important and private note to her friends is because the dope is too short-sighted to understand or listen to her side of the story.
This leads us to the most ruinous thing that occurred to a character because of his inability to see past his nose or to understand what that weird word "consequence" his uncool parents use when they try to stop him from doing a cool, good thing means: his setting Lawrence up to have his life ruined because his sense of right and wrong won't allow his friend to keep a very big and very damaging secret long enough to sort of defuse it. Just as he's going to spend his life bleating about how it's cool that he treated Liz shabbily because having a happy kid sister is wrong, Mike is probably still saying that it's a great and glorious thing that he drove a wedge between Lawrence and Connie because nothing is ever his fault. It's like I always say: being stupid means never having to say "I'm sorry."