Yes, TestFlight is absolutely great for beta testing.
However, in the startup sense, it's not exactly beta testing, it's testing the market, so beta testing would be fine to test something you've built works, but by no means will show what people will think of the app/product.
However, in the startup sense, it's not exactly beta testing, it's testing the market, so beta testing would be fine to test something you've built works, but by no means will show what people will think of the app/product.