What seems to be happening with the Pattersons seems to happen for two reasons. The first seems to be that she and the kids have wildly different ideas of what should be worn in the first place. I should tend to think that Elly's idea of what constitutes perfectly good clothes for Modern Children fossilized itself in the form of the rather unflattering ensembles we see preserved for the ages by James Lileks. Given who Elly is, it's not much of a stretch to assume that she'd see her children not wanting to get punched out or laughed at for what they wore as a cruel, vicious and horrible rejection of the love in her great big heart.
While all three children have this issue in common, Mike has an extra incentive to hate trying on clothes for school: school. To him, school means having to do things that confuse and anger his empty brain like figuring out the impossible question "two plus two is what?" School means exposure to evil monster teachers who clearly want him to suffer because the alternative is admitting that he's a lazy dimwit who wouldn't accept the answer if they handed it to him on a silver tray. School means exposure to social situations that remind him that he's not cool, he's not really the center of attention and he's not especially respected or liked all that well. School means accepting the horrible truth that the point is exposing what he doesn't know so they can fill that void with knowledge and also the horrible truth that he's no big deal so he can go through life without bleating about how noooooooobody loves him if they ask him to pitch in. Finally, school means that the source of most of the problems in his life is currently staring him in the face when he shaves.