I absolutely agree, we cannot judge a person based on what they are coming to the Holy Sacrifice of the Mass dressed in. However, let me ask you to approach it this way:
The Mass is not an event where we come to express ourselves, but where we come to commune with our Lord and Savior. We are coming to his banquet, the Holy Wedding Feast of the Lamb of God with His Bride: you and me and the whole Church. We are coming to participate in a Divine and Heavenly Sacrifice. While you share with us that God did not “tell the rich man he had to dress in his Sunday best,” he did in fact severely chastise the man who came to the King’s Wedding Feast for His Son, dressed unworthily Matt 22:2-14; Lk 14:16-24.
While this parable applies to spiritual matters, the literal approach is also significant, if you have the ability to dress nice in respect for what you are participating in, than you are obliged to do so. Willfully dressing in a sloppy, immodest or poor manner when you have the ability to dress worthily reflects poor theology and an ignorance of the significance of the Liturgy one is participating in.
If, God forbid, tomorrow a loved one should die, would you not want to dress in your very best at the funeral which is held IN MEMEORY or COMMEMORATION of your loved one? Or at a very special wedding, say you were invited to the wedding of a great leader (I know, we are greatly starved as a planet in this arena, but imagine a great President was getting married and you were invited), you wouldn’t dress like a slob or immodestly, would you? Now realize that at Mass we are celebrating and entering into the Life, Death and Resurrection of the King of kings, Lord of lords, of the greatest Gift of the Eternal Father: Himself.
I challenge all people who think it is right and fine to dress down for the Holy Sacrifice of the Mass, to begin to set aside their own wills in this matter, and to seek rather to add something to the Liturgy of their Lord and King by dressing up for Him, showing Him with your bodies and the choices you made from the moment you got up in the morning to the moment you picked out what to wear, that you were going to make an effort to glorify Him with all that you have. Do this, and you will not fail to receive more out of the Blessed Sacrament, because you are making yourselves more open and receptive and cooperative with the grace being extended to you. When we dress nicely for the Mass and show all reverance, it is also a powerful act of evangelization and catechesis for the newcomer and the lapsed Catholic. It adds to the Liturgy by showing everyone else that what is happening is worthy of all respect and awe, and it acts as a temporal or material aid, in a way a sacramental, in helping the entire congregation gathered for the Mass to spiritually participate in a worthy manner.