I can feel your frustration. Unfortunately Shopify as a platform and a theme your website is using are two separate things. While Shopify makes a set of functionalities available, how and whether a theme will utilize them is up to a developer.
And to defend the developer - the functionality is there, it’s just the way it was designed for mobile phones makes the cart icon being displayed different way. I admit there could be a number of items next to the icon, but current feature shouldn’t make the whole theme non-functional.
Thanks
Lukasz