To be fair, Agnes' transformation only came about because of Kelly and Nazelbert. If she had married Yarata, she would still be under her family's thumb and wearing those unfashionable clothes. Though that probably would still be better for Yatata than his current situation.
Mother was talking about traditions and that lady needs to persevere, even though clothes is heavy... but turns out that mother was cheating with her magic all this time. Wow...
I know that this kind of clothes have more issues than just weight, but still...