The buttons are just too tiny and too close together, making the UI unusable.
To see for yourself, run it on SXMO (if you can) or set screen scaling in Phosh from 200% to 100%.
I'm not familiar with all possible techniques for possibly fixng applications, is there an overview somewhere? I found GDK_SCALE, which only works for GTK applications. I guess DPI can only be configured for the entire screen?
Also a question that came to mind, is there some sort of policy to develop applications for this 2x scaling factor rather than the devices native resolution?
Megapixels is a GTK application, so
GDK_SCALEshould work just fine. It's not designed for any specific resolution, but it uses normal-sized UI elements just like every other GTK application. It looks tiny without screen scaling because the display is small.
Also scaling and other GTK things are a bit weird in sxmo because gnome-settings-daemon is not running, which provides gtk apps with theming and display info normally.