I’m often accused of hating Flybe (which I don’t) but I have to agree with the others. Why do they need an app?
Their website works perfectly well on a mobile. They don’t have a frequent flyer scheme or similar so no other real reason to need an app.
I’d say the lack of frequent flyer scheme or ability to use another is holding them back more than lack of app especially when up against IAG on routes from Belfast.