Question: Is it appropriate to daven in shul on Purim while wearing a fancy-dress costume?
Answer: Davening nowadays has replaced the korbanos that were offered by the kohanim. Just as they wore unique clothes when they performed their avoda, it would be ideal to have separate clothes for davening in. Nonetheless, the Shulchan Aruch (OC 98:4) concedes that this is not really feasible.
Rambam (Tefilla 5:1) lists appropriate dress as one of eight things that are necessary for davening. The Mishna Berura (91:12) writes that one should dress as one would when greeting a dignitary, noting that what is considered appropriate is different in each place.
The Shulchan Aruch (OC 90:23) writes that one should not daven in front of art or colourful drapes as they can distract one’s davening. The Rema adds that one should avoid printing pictures in siddurim as they can also be distracting.
Thus, R’ Shmuel Wosner (Shevet Halevi 10:18:1) writes that one may daven on Purim while wearing a fancy-dress costume, though one must ensure that it is respectable, properly covers the body and will not distract themselves or others. While the example that R’ Wosner brings is of one dressing up as a chassid, etc. it would seem that this would be subjective to the shul that one davens in and what would be considered appropriate there on purim.
In conclusion, one may daven in shul on Purim while wearing a fancy-dress costume, provided that the costume is appropriate and would not unnecessarily distract anyone from davening properly.