Been there, done that.
Her Mum may very well be bad mouthing you. You can't do anything about it, ignore it. Don't rise to the bait or retaliate by bad mouthing her. However you are tempted, NEVER badmouth her Mother to her. It achieves nothing except more conflict.
In our early days I got lots of "Mum says you're going to leave Dad" "Mum says your're a snob" "Mum says insert random insult" Each and every time I replied with "That's interesting" or "Is that so?" or "That's really sad that your Mum feels so unhappy to be saying things like that". It stopped pretty quickly. I also had "I want Mum and Dad to get back together. I don't want you here". The fact that her parents had split long before I came on the scene was irrelevant. I was unknown and and therefore a threat to the status quo.
We'd buy a picture to hang on the wall, the topic of how Mum was a great artist would be raised. I'd wash dishes, it would be mentioned how Mum preferred to do it this way or that. I'd do a roast dinner. Mum was apparently an expert in the kitchen and her food was "to die for". Again, I just responded with "That's interesting" or "It's good we all have our own way of doing things" or "I admire people who are able to take raw ingredients and make an amazing meal" etc.
From my experience I'd hazzard a guess that she's scared of how you will impact on her life. She's worried that if her Dad loves you, that means there is less love for her. She's scared she's going to loose him. Dad's girlfriend is one thing. Dad's new wife is a whole different ball of wax. As she sees it, her whole world is turning upside down and she has no control over any of it, and that's very frightening.
Your partner needs to reassure her, lots. Over and over. Tell her that he loves her, that marrying you will not change that, nothing can change that. He needs to tell her that the love he has for you is very different from the love he has for her. My DH used the analogy of cars and ice cream. He said he loves cars, they are his favorite vehicle in the whole world. He also loves chocolate ice cream. Its his favourite dessert in the whole world. He can love cars and he can still love ice cream, because they are different. He needs to tell her that she is his number 1 girl, his daughter, and always will be. You are his number 1 woman, his partner, and that's ok because it doesn't stop her being his no.1 girl. You're different and the love he has for you both is different. He has a big heart, there is room for you both etc.
It's possible her Mother will say that it's ok for her daughter to like you. Unlikely, but possible. Personally I wouldn't hold your breath. Sadly it seems to be the norm for a lot of ex Wives to enjoy the fact that they can cause tension in the new blended family and not care about the damage this does to the kids. Again, this is something your partner needs to tackle. You can't. He needs to tell his daughter that he understands how confused she is and that it's ok for her to like you, and still love her Mum.
He also needs to tell her that you're not her Mum, and it's not your intention to replace her Mum. She has a perfectly good Mum, she doesn't need another one (that may not be true - her Mum may be the worst Mother in the world, but her daughter does not need to hear that). He also need to say that even though you're not her Mum, but you are one of the adults in your house and that alone demands respect and good manners. You don't have to like one another, but you do have to live together from time to time.
I also agree with getting her involved in the wedding where possible. Coax her into it gently. She's going to be part of the blended family, so getting her involved in the "fun" stuff and having a special role in that is really important.
Hang in there, you're doing great. 