ISO 27001 is about how security is managed, not how secure a product is. It would, therefore, be very difficult to define a particular app as the scope for certification if the infrastructure is also under the company's control.
You manage the security of the infrastructure and then develop on top of that secure foundation. That's the intention of the certification.
If your scope of control is only the development of the app, then you still certify the development company, not the app.
That isn't to say that you couldn't convince some assessor somewhere to do certify you with just the app as the scope. I'm sure you could. But any potential partner/customer/stakeholder looking at the certification's scope will not take the certification seriously and start to doubt your competence more than if you had no certification to begin with.