ZUN loves to give a vague power to Touhous, never fully expanding upon the limitations and ranges of the powers. Often, it's up to fans to extrapolate what a character is capable of.
Cirno's power is about cold, ice, etc. She's just an idiot about it.
Density, in Suika's case, is not about stupidity. Rather, in observing her powers in IaMP, she controls volume and mass. This is shown in various doujin and in the game with her growing very large or very small in size, as well as changing into mist (changes in volume). And, with higher densities, a greater force of gravity. This is also how she draws everyone into a giant party in the events of IaMP, as well as certain moves in the game like a gravity well. Additionally, her support ability in Subterranean Animism is to draw all items to Reimu, likely via gravity manipulation. And if we extrapolate this manipulation of density and gravity to extremes, Suika can theoretically create black holes.
She's also strong enough to lift a mountain. That's pretty darn powerful in its own right.
