I think how a person dresses (who isn't unduly constrained by poor finances) on a Sunday morning at church reflects something of their heart toward the God they've come to worship. Those who have diminished God in their thinking, who do not see Him as the awesome and terrible Creator of the Universe, as the Ground of All Reality and the Sustainer of Everything, as the holy and perfect Jehovah who met with Moses with thunder and lightning on Mount Sinai and who filled the temple with devastating brightness, tend to take Him lightly and deal with Him with a casualness that has become quite grotesque (ie. flip flops, tank tops, beach shorts, sweat pants, etc.)
Many in church on Sunday aren't really there because they are interested in God. They've come for the show: music, lights, food, socializing, motivational speaking. They worship God with their mouths, rocking the house with Jesus and the worship band, but their hearts are from their Maker. And it shows. Why dress for a meeting with the Queen of England when you're going to a rock concert?