You could log out of the linked devices, and you cannot log out of Mixin Messenger which runs on Android or iPhone.
Log out from the linked device
Tap the avatar at the top left corner; then, you can scroll up to the Sign out option.
Sign out the linked device from the Android or iPhone
Tap the Desktop icon on the top
Tap the avatar and get into Settings > Mixin Messenger Desktop
Then tap Log out from desktop to confirm the operation.