I do use a 24" monitor with 3840x2160 resolution. This is just a bombshell ... the finest your eyes can experience. And this perfect little tool, set to factor "1.6" is a convenient way to have a useful viewing experience. Without this adjustment even my 125% eyes have severe difficulty. May be you could just adjust layout.css.devPixelsPerPx but this way is more convenient.
Long answer: This addon has been developed using the new Firefox Addon SDK. As far as I see it, porting the addon to Thunderbird means rewriting it without the SDK, which it currently heavily depends on - both in code and in the tooling used to package it.