It depends on the type of business your startup will sell for.
For B2C startups to have an APP it is a must. No way you launch your MVP without having an app.
For B2B startups the situation is different. Depending the type and size of business you will sell an app is only a nice to have in the beginning of the company.
I suggest you to check for competitors (most successful) in your area and discover what was the product roadmap strategy. Either mobile or web.