A reasonable investor is going to want more than just mockups. And "mockups" is a wide ranging catch-all. We typically build out high-fidelity, production-ready prototypes in Invision so that they're wired together so clients can "demo" the product to prospective investors. But even still, investors want to see a business plan, an understanding of the costs that go into building the product, marketing the product, running the business, scaling the business, etc, etc, etc., and mockups don't give enough.
There's a whole spectrum of "minimum viable product" but as far as investment is concerned, the further along you are with being able to demonstrate proof of concept and proof of audience, the better valuation you'll secure and the better chance you'll get an investor interested.
That being said, investors have definitely sunk seed money into just the person running the thing from the scribblings on a cocktail napkin they showed off.