This is possible by overriding our mobile template for the topic list — the customization shared here still works (this can be added to the head section of a theme): On the mobile theme, the list of posts show's the latest replying user's avatar, not the OP's avatar.
Though one follow-up question… the screenshot you shared appears to be the desktop version at a narrow width, Discourse on mobile looks like:
If you want to preview the mobile view and you’re not using a mobile device, you can add ?mobile_view=1 to the end of the current URL.