Is usability (UX) important for a startup?

Yes. Always.

The difference between startup environments vs. traditional businesses (ie manufacturing at General Electric (company)). is that startups have the luxury of "figuring it out" over time, via rapid user feedback and constant iterations, whereas traditional businesses have to place more emphasis on finding the right answers during the conceptual, early stages of product development (focus groups, surveys, etc).

For rapid usability testing you can use: