Again, it’s going to extremes. You don’t have to dress like a late Queen to look elegant.
I just don’t think a polyester dress (sorry, but it looks cheap) is the right choice for a mother of the bride. For a 20-year-old cousin who wants something for a night out later on, sure.
Style isn’t about money. You can find something for £59 that isn’t plastic and still looks elegant.
Personally, I don’t think this fits a wedding for a mother of the bride. She asked for opinions, and this is mine.