Most pastors will, sadly, say nothing, accepting to the platitude that "God's just happy that they're here." Given Jesus' words about the wedding garment (Matthew 22:12), I don't think that's true.
I like to suggest that we should not wear to church what we might wear to bed or to the beach. It seems simply enough, really. Perhaps you are going to the beach after church; I'm sure the church has a restroom in which you can change afterwards; if not, the beach surely does, or a gas station on the way.
Over at Adam's Ale, Father Valencheck offers his own thoughts on the subject:
On the other hand are those who know and can dress better for Mass. The idea that God loves me “as I am and so should you” is an inherently selfish one. It is not about you. It is about us. And part of being “us” is showing respect to those around you. The reason we dress up for a potential employer or the judge at our trial is because it shows respect for that person and a willingness to be part of the goings on. We do not expect them to “take me for who I am,” an absolutely ridiculous thing to do.So what to do? There was a group of young people in Cleveland that thought people should be dressed up for the opera. Instead of passing edicts they went out, got very nice clothes, and started setting the bar higher themselves. So the first step is to take care of self and family first. Show that it is possible to survive for an hour or so in the summer in one’s Sunday best.
The second is to not too quickly take others to task who look like they are rather going to a beach party. The reason we hear of no clothing disputes with Jesus is that he first got to know people and then invited them higher. Imagine the difference in these examples: 1) The look of “how dare you dress like that” and the loud whisper to another saying, “how could s/he come dressed to Mass like THAT?” or 2) Getting to know someone and saying, “Next Sunday we are going out to breakfast after Mass. Why don’t we all get dressed up and go out together?” [more]