This is likely a quirk of running in the anaconda environment – it expects to be set up correctly before running, and the startup scripts for anaconda are typically invoked from your shell’s startup scripts (normally ~/.bashrc). The simplest way to modify your desktop file is probably to start mrview within its own shell, which you might be able to do by replacing the Exec= line with something like:
Did not work with that alone, but after spending some time I made it work. I realized that when launching the application the context menu showed the option “Launch using Discrete Graphics Card”, and it would be only when clicking on it that the application would start correctly after adding it to the dash. So after some research I found this post.
Also, in order to launch the application correctly from the dash, and prior to adding it to the dash, I need to execute: