Well, I think you hit the nail on the head when you said "I am the mom and what I say goes." YES, you need to say exactly that. As long as it comes from a loving place, you are being honest and more importantly, you are putting your childs needs ahead of your need to not hurt your parents. You need to be firm with your parents and explain to them how important it is that she sleeps at nap time. Do you have a regular schedule for her? If so, try to have them before or after her nap time, and if they are all day visitors, then they need to go out for lunch or help you around the house while your daughter naps. You are her mom, and no one knows her like you do. I know how important schedules are to little ones, and your mom should too. Tell them how thrilled you are that they want to be such a huge part of your daughters life, and that you love the bond they are creating. But, they need to respect your parenting style, and support you. I am the only one out of 3 girls in my family to have children, and my mom was ecstatic about my daugter when she was born. She still is after 9 years!!! My mom was with us almost every day to just adorn her with love. The bond they have is such a special gift and I love how close they are. Nurture that, Grandparents can offer so much to our children, but they must respect the way you do things at all times. We all have different ideas on parenting, but this is your baby and you have the right to raise her however you seem fit. I whole heartedly agree with you about not holding her while she sleeps, its one tough habit to break. Tough on you and your daughter.
I believe that honesty is the best way to handle this. Tell your mom how you feel, she may not realize just how important this is to you.