Hey there! We're aware that the comment-collapse animation is a little jarring; this is something we want to improve in future.
You can simplify the animations used when switching pages and when opening the image viewer by going into System Settings -> Accessibility -> Per-App Settings (right at the bottom) and enabling "Reduce Motion" and "Prefer cross-fade transitions" for Mlem.
Disabling the comment-collapsing animation isn't yet possible. I've opened an issue for this on our GitHub, and will look into disabling it when "Reduce Motion" is enabled 👍