So why is Braum allowed to show off his well toned body while Sejuani is not?

You can't argue that Sejuani should wear armour because it's oh so cold in Freljord because they live in the same general area. You can't argue Sejuani being a tank requires her to wear armour because it's Bristle who is taking the hits and not her...plus Braum is a tank himself! They're both barbarians, yet Braum is the only one who actually looks the part.

Why aren't there female equivalent of white knights calling for Braum to cover up his shamelessly flaunted shameful body?

And don't even bother bringing up the fantasy fullfillment argument, because anyone whose talked to a girl in real life know they aspire to look as good as Caitlyn and old Sejuani just as boys fantasize about being like Tryndamere or Braum.

(similar argument can be made comparing Tryndamere with Sejuani)