לע"נ ר' מרדכי גימפל בן ר' אברהם משה
The prevalent custom is for people to daven on Purim while wearing their Purim costumes. This would seem problematic because one wouldn't stand before a king [or the President of the US] in a clown or other type of costume. The Mishna Brura says that when one davens the clothing he wears must be fitting for appearance before a king.
Rav Vosner ztz"l [Shevet Halevi 10/18] say that if a non-chossid is dressed like a chossid or vice versa, it is permissible because one would wear these garments before a king.
אשר שאל בענין אשר הרבה נוהגים בתחפושת בפורים החלפת בגדים כגון ליטאי עם שטריימל או להיפך, אם מותר להתפלל בזה תפלת מנחה או מעריב עפ"י המבואר או"ח סי' צ"א שצריך כדרך שעומד לפני אנשים חשובים. הנה לדעתי אין חיצונית המלבוש קובע אלא אופן עמדו לפני השי"ת בתפלה עפ"י המבואר סי' צ"ח דיחשוב כאלו שכינה כנגדו ועומד לפני מלך העולם ועוד וכיון שמתפלל ע"פ הלכה והוא מכוסה כהלכה ואינו עושה שום שחוק אין נפ"מ באיזה בגד עומד
The clear implication is that if it is not a honorable be-kavodike garment, one may not wear it.
After asking mechila from one of the gedolei ha-dor, Rav Vosner ztz"l, I will make 2 comments.
1] On Purim, one WOULD appear before a king in a Purim costume. You see many chasidim going to their Rebbe's wearing their purim costumes and you see Bnei Yeshiva going to their Rosh Yeshiva in their Purim costumes. So PERHAPS Purim is different.
2] He seems to hold that it doesn't matter what this particular person normally wears. As long as it would be considered for a others a בגד של כבוד it is fine.
If that is the case then one need not wear a hat and jacket when he davens [which is de riguer in the Charedi world]. Since in many circles they do not wear a hat and jacket and even if the greatest king or li-havdil a gadol bi-torah would come they would wear a white shirt and dark pants but no more, then it is OK for everyone. That is hard to swallow [לא מסתבר in talmudic parlance].
It would seem to be a subjective issue based on what THIS PARTICULAR PERSON wears and if he normally wears a chasidic hat then wearing a non-chasidic hat is not appropriate for davening or vice versa. A kibbutznik may daven in sandals and a t-shirt because that is how he would appear before BN [the PM of the memshala ha-tziyonis] while for a member of the Chazon Ish Kollel it would be a terrible breech of respect to daven this way.
Just some thoughts to stimulate discussion.....