Wikipedia Categories are a navigation feature and as such deserve to be displayed for mobile users. Thanks.
I'd say for your average reader, categories and the app's "read more" feature serve the same user story: "As a reader, I'd like suggestions as to what I could read next once I'm done reading my current article". I suspect "read more" does a better job since it's more compelling, but we don't have meaningful data for categories on desktop so we can't really know.
In the long run we should rectify that we have two totally different systems on desktop and the app for giving reading suggestions.
Right now, this task is not a priority for us at all.