WPPizza is a wordpress plugin and not a stand-alone cart/shop/ecommerce solution.

The only way (perhaps and as far as I know) to make WPPizza into a mobile app is to convert your whole wordpress website into a mobile app using – possibly – something like these plugins/option listed here https://premium.wpmudev.org/blog/wordpress-site-mobile-app/ (or other resources that you might find elsewhere on the net)

There are a few plugins out there that claim to be able to do this sort of thing, however, I cannot say anything in particular – good or bad – about any of them.

PS: If you should find something suitable for this purpose, I would be happy to hear about it to pass it on to others.

Update [16-08-2018]: It appears that a mobile app incorporating the WPPizza plugin has been developed (see https://www.yemekuygulama.com/en/ ).
However, please be advised that listing this link here is for information purposes only and does not constitute a specific endorsement of said app.
I have not tested it or are indeed able to tell you if it would be suitable for your purpose if you were to use it. It is simply to say that mobile apps ustilising th eWPPizza plugin and its extensions are seemingly possible.
Please direct any and all questions regarding this particular app to the developer directly. Thank you.