I've got a moss wall growing in a 2.5, so it is definitely doable. "too much" of the tank depends on what else you want in it. it restricts space, but can make a great atmosphere.
In my 2.5, I have only mosses and an anubias nana 'petite' as an accent: christmas moss (back wall) java moss (side walls and floor) and peacock moss + stringy moss on my one rock hardscape.
I suggest using the craft-square stuff and not the guttering cover material for such small tanks because the former is stiffer easier to keep tight against wall, which maximizes your space (I tried the guttering cover material and all my shrimps hang out behind it. With the craft-material, they can't get back there.