If you want to hide the topic list below the category specifically on mobile, you can do this by going to the Edit HTML/CSS section and adding the following under the Mobile tab:
.category-list-item tr.category-topic-link {
display: none;
}
If you’re unsure how to edit the HTML/CSS, you can visit https://yourdiscoursedomain.com/admin/customize/components. Click on Install, then + Create New. Name it anything, set the type to Component, and then open the file you created. On the right, you will find the option to Edit HTML/CSS.
Also, make sure to enable that component for your theme. For more details, you can visit Making Custom CSS Changes on Your Site.