Yeah I noticed that too!
Though maybe's it's not supposed to Actually look good, as it's something she herself chose, because she wants to wear it. I assume next chapter is going to start with the fashion designer disagreeing with them or something along the like, and the only reason Elma says it looks nice is because he can tell she wants to wear it, so he doesn't want to ruin it for her. That or you're right.
Like look at the face he makes on page 14, that's not one of awe, that's one of 'Really? that one?'
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To be honest I think she even looks prettier in her jester outfit compared to the one she chose, she just needed to take off the hat.