The bottom line is that overly casual dress must be firmly resisted, because it takes over if it is not resisted.
Overly casual dress invariably leads to the appearance of a lack of reverence.
Bingo! Good thing this thread is not on FE as a majority of the posters have proved themselves clueless on this topic. No doubt a majority of them would have already mercilessly accused Spouse. Tele. etc. of being judgmental jerks and the thread would have been locked due to lack of charity and the OP banned.
Again it is with a sigh of relief that I am able to actually have an intelligent discussion of this topic with Catholics whose Catholicism does not exclude all externals as if they were evil. This is not to say we will all agree, but at least the discussion is with those who are sane and get it rather than imbeciles who can't distinguish discussing proper attire from condemnation of them as individuals (most likely because they dress like slobs for Holy Mass).
In any case, the key points here are reverence, modesty, and respect for Our Lord. Casualness and familiarity in dress is to be avoided as a rule. As has been pointed out, this doesn't mean wearing a tux to every Mass, but it does mean wearing something that shows respect. I doubt any of us would go to a business interview in a t-shirt and shorts out of human respect. So we should at least make an effort out of respect for God.
Coats, ties etc. are up to the individual, but I'd say minimum you'd want slacks, khakis, dress shoes, button down shirt for guys. (Even if you are dirt poor can you not find khakis for a few bucks at good will? Jeans are often more expensive) If you live in a tropical/ warm climate collared polo shirts would be acceptable I think. But again, this would be the minimum. If someone wants to wear a suit, more power to them. Most wear a suit for work every day anyway.
I continually fail to see how something so simple, showing respect for God through dress at mass is so vehemently attacked by Neo-Trads and others. They will defend the most egregious offenders of basic respect at Mass rather than agree that there is something wrong.
Of course you don't go yell at someone who dresses inappropriately, but hopefully the congregation is dressing properly enough that they easily see they are out of place. If the congregation is clueless and thinks "God doesn't care what I wear, He better be glad I'm even here." then the priest needs to give them a wake-up call. The state of most NO parishes is so atrocious that parishioners are frequently showing up in shorts and t-shirts.
Can you imagine this practice in a Traditional chapel? It is almost like slapping God in the face to me. "I could give a crap, I wear what I want, you're lucky I'm here." comes to mind.
The terrible lack of reverence embodied, in part in the dress of
NO parishioners is one of the factors that made me realize something was wrong and inspired my journey to Tradition. Externals do matter.