I don't know how your Kingdom Halls work but many Catholic and Episcopal parishes are open all day for people to come and pray in the sanctuary or try to speak with clergy. They usually have signs out front saying "all are welcome" or something similar, not "all are welcome unless you don't fit our standards of dress". At the Episcopal parish I attend, there might be a couple homeless people sitting or sleeping in the pews during and around the liturgy. No one cares and they don't bother anyone. Yes, they are dressed in shabby clothing, too. But they are still God's children and it's better they be in a church then on the street, even if it's just for a few hours.
No one is turned away from a Kingdom Hall. Everyone is welcome to enter. Many people who have come for the first time come dressed the way they are used to dressing in the world. It could be as a biker, as a bum, as a warlock or witch. Friends and relatives of Jehovah's Witnesses come to meetings all the time dressed in play or casual attire, jeans, t-shirts, everything you can imagine.
When one is serious about worship then they realize that their appearance affects not only onlookers, but also their relationship with God. So a Jehovah's Witness dresses modestly, but neatly out of respect of their neighbors, and of course out of respect for the worship they give Jehovah God.
Here is a simple video from a member of the governing body of Jehovah's Witnesses explaining why a worshiper of the true God should use proper dress and grooming. The points basically are:
1. The wrong type of grooming can give the wrong impression on a Christian's morals and principles. - 1 Timothy 2:9.
2. Whatever we wear should in no way be a cause of stumbling for another person, believer or unbeliever. - 2 Corinthians 6:3.
3. When selecting what you wear have in mind what God wants. Will this raise eye-brows? Will it lower the standards I have? Does it fit? Is it appropriate for my age? Number 15:39.
4. Is the clothing upbuilding, does it represent Jehovah God in a fine manner? Would someone be shocked to know one is a Jehovah's Witness if they saw the way they dressed. - 1 Corinthians 10:23, 24.
5. It is good to look to mature Christians and their standards for dress and grooming if we are perplexed as to how to dress. -Philippians 3:17.
Jehovah's Witnesses do not dress to impress or to be ostentatious. They don't have a separate clergy class that wears special garb either. They dress neat and groom well because they believe the way they dress and groom reflects their God, and their standards. And anyone is welcome to worship with them in the dress they are comfortable with.
So as to your question about whether can a bum who stinks and wears rags come to a meeting? Yes, we won't turn them away. If they are serious about serving Jehovah they will clean up though. And they no doubt will be helped. I myself have given away clothes, suits, ties, shirts, dress shoes to people wanting to dress more appropriate that are coming to the meetings, as have my wife, and I expect most other Jehovah's Witnesses that are out there.