I believe Michelle's determination and work ethic earned her preferential treatment moreso than her relationship with the Undertaker. She turned into arguably the best women's wrestler to come out of the WWE's Diva Search, so I could see why Vince preferred a home grown protege like Michelle McCool over wrestlers with prior experience like Mickie James and Melina. I will say that I think it would be a little naive to profess that her relationship didn't have some sort of role for her push, but even with her work ethic, Michelle was slut shamed by the IWC for her push. That was the case with a lot of model-turned-wrestlers like Alicia, Eve, and the Bellas.
That is a very good point that I think people often forget. Vince may have had more reasons than just "Oh it's Taker's wife and I like him" to push Michelle. Just like Trish Stratus, she was an internal talent. WWE has a very strong history of fucking over people that they don't consider home grown stars in favor of people that grew up through their developmental system.