The descriptions of pinned topics (and the category descriptions) aren’t shown on the mobile category page because there’s a limited amount of space and we’ve prioritized topic titles instead.
By default we only have one mobile category page style, which only shows categories and the 3 latest topics under each:
It’s possible to choose a page other than categories (like /latest or /top) for the mobile homepage using: Force Mobile Homepage
You could also create a custom category page for mobile by overriding the mobile category page template in a theme. Some theme-related guides:
Beginner's guide to using Discourse Themes
Developer’s guide to Discourse Themes