Q. Can one wear a Purim costume when davening?
A. Shulachan Aruch requires that one davening the Amida, to be dressed properly as if he would be standing facing a king.
Therefore, Horav Shlomo Miller’s Shlit’a opinion is that one should not wear a costume at that time.
Rabbi A, Bartfeld as advised by Horav Shlomo Miller and Horav Aharon Miller Shlit”a