I'd argue an ImGUI does't mix anything. It's just a library. If you want MVC it's up to you to implement your MVC and use your ImGUI as a library. Retained mode GUIs don't magically make your code MVC and ImGUIs don't magically make your code not MVC. That's entirely up to you.

Also ImGUIs are not multiple pass. That's in implementation detail. Unity's ImGUI is multi-pass. One of the most popular ImGUIs (Dear ImGUI) is single pass

https://github.com/ocornut/imgui