In Victorian times, a matron was an older, married woman of some position in the community who would be expected to conduct herself in a dignified manner, unlike the younger, unmarried women who could be more flighty and irresponsible. A matron's clothing would be a reflection of her modesty, tastefulness, and position in the community.
As modesty, tastefulness, and dignity are probably NOT the goals of most Project Runway designers, I'm thinking that calling a garment "matronly" is not much of a compliment.:0
*giggling at the thought of a Victorian "matron" in her puke-green floofy nightgown*