Should do since it's a Pokémon that can still evolve. I think Munchlax have good defences don't they? Eviolite will give it even better defences though of course, at the expense of using any other items like Quick Claw say.
Munchlax; cute Pokémon.
Depends on levels and stats. The base stats for Munchlax's Defence and Special Defence are 40 and 85 respectively while Snorlax's base stats for Defence and Special Defence are 65 and 110 respectively. It may have a beneficial or hindering nature which may affect these stats if you had two different Munchlax say. But in any case, a 50% increase on both would give a theoretical base stat of 60 and 128 (rounded up slightly for convenience) for Munchlax as long as it's holding Eviolite. Of course, there are other stats to take into account if a Pokémon evolves but unevolved Pokémon learn moves quicker and moves that may not learn when evolved. They probably are better off stat-wise when evolved in most cases but as for me, I love Oshawott too much to let it evolve so Eviolite gives him a good fighting chance.
Hope that helps!