I think YABU to complain about her being too thin but not to complain that there should be some variety in size/shapes of models used.
For those arguing that they should use size 12/14 that doesn't really solve the issue as they would still only be showing one type of woman. Probably also unusually tall to make them seem as slim as possible for that size.
I would think some taller models as well as shorter ones and a variety of shapes and sizes would work best. Obviously this is a big ask for each individual item but I think if the entire online catalogue was more varied more women would appreciate it.
I'm naturally thin and find it offensive when people essentially body shame other women for being thin. It's no longer acceptable to complain that women are "too fat", the same should be true for thin women.