I have a different perspective since I became a fan in 1970 when Jean started with the Supremes and Diana Ross was a soloist. I liked them both for different reasons.
I was drawn in with Jean's voice but was unimpressed with her television appearances. She changed her vocal approach with the Touch LP which had a different sound than the Supremes, more full bodied and less driven by a danceable sound. She rebounded with Floy Joy, aided by Mary. Then with Jimmy Webb, she sounded shrill. I had a friend watch a You Tube video of JML doing Bad Weather and they stated that Mary seemed to be trying so hard to garner excitement but they felt this trio did not have "it". They called Jean "that nasal thing" and wondered who Lynda was saying they felt she didn't add anything to it like Flo and Cindy did. Their opinion not mine and they only know the Supremes very marginally.
I always like Diana Ross although she could be too over the top and too theatrical in look and performance. I never really judged her as a Supreme because that was before my time. She may be an icon but somehow the media and general public don't seem to be as crazy for her as say they go for Streisand, Cher or the like. She has a distinctive, pliable voice but it is very thin.
I have always liked Mary Wilson's voice and look but am well aware she has shortcomings. I think all of the group had talent but they all have shortcomings, even Ross but no one points to others, only Wilson. Mary's voice was deep and not a soprano and that was needed for the sound and it has a great deal of warmth to it. I figure Motown didn't like her because her voice didn't lend itself as easily to the danceable Motown sound. Still, she could have been Motown's female balladeer. Diana's voice could be weak, Flo was too loud and unstable, Mary had limitations. None were perfect. Lots of fans and general public wanted them all to sound like Ross but as a group that would have been horrible. Imagine a trio of Diana Ross, Jean Terrell and Susaye Greene.
Yes, Motown did not like Mary's sound but many fans wanted her to sound like Diana because they liked her and her voice. I accept that as much as the fact that I accept liking Mary's voice and talent as well.