I wonder if that's what it is? Curse those pesky Americans and their overly ornate mediaeval style bedwear! 
The problem is that it's rather bland. It doesn't got any color, no ornamentation, nothing. It doesn't look really medieval and looks particular out of place next to the incredible outfits of your advisers. These outfits looks gorgeous while the Inquisitors outfit looks pretty bland.
Why people are saying it looks like Pyjamas?
Bland Color, skin-tight outfit and when you don't pay really close attention it looks like it's a 1 piece-outfit. Like Pyjamas.
But as i allready said a big part of it is the bland color, that just doesn't fit to a person, that's supposed to have monarch-like power.
If they would have just given it a more regal color, like velvet red for example, it wouldn't be that much of a problem for people.