How can I cut a (reverse) waterdrop shape out of a cardstock perfectly?
If you have a makerspace in your area or a similar craft related activity/club, you may be able to gain use of a craft cutter such as the Silhouette Cameo or a Cricut product. Still associated with the makerspace, you may find members with a laser cutter. Stretching the application a bit means you could also use a flatbed CNC machine to make the straight and large curves, although the small point at the bottom would have to be done by hand with a sharp craft knife.

I've recently become aware of a product that can be best described as a high-frequency vibrating craft knife. At a cost of US$399, it's steep for hobby use, but might be practical for high production craft use. It's a Kickstarter campaign called Wondercutter, but as a Kickstarter item, there's a risk involved.

As I own a laser cutter which cuts cardboard quite cleanly, it's on the top of my suggestion list.