Obviously Empress Frederick, who was an implacable enemy of the 'fringe', wasn't able to exert enough influence to talk her out of that on her wedding day.
I think it really made poor Margaret look rather frumpy on her wedding day. The dress was a beautiful, if overdone by today's standards, Victorian concoction but her hair looks rather blowsy.
Yes, mothers traditionally helped with the dress (they probably had more influence then than today) and definitely with the trousseau since it would contain 'grownup' outfits and unmentionables. In the case of Princess Alexandra, her mother would've had sway if not for the fact that she was marrying the heir to the British throne and Queen Victoria's wishes (as per usual!) took precedence over a mere 'Princess mother'.