Will lightly whipping the cream then folding it into the rest of the pumpkin pie mixture make it light and fluffy or ruin it?

Hi Heidi! Are you talking about after or before the pie is baked? I wouldn't fold whipped cream into the pie before baking it—that's going to mess with your results. Instead, I'd go with a recipe specifically tooled to give you fluffier results: http://cooking.nytimes.com/recipes/1017820-julia-childs-aunt-helens-fluffy-pumpkin-pie. In this one, you achieve the fluff by folding in whipped egg whites (so you could also apply that same technique—separate the eggs, beat the whites to stiff peaks, then fold them in—for a lighter pie).