>>57872353something with no useful niche in the campaign or competitive (singles or doubles)
aka mons you'd only use as a self imposed challenge because they're worse in every aspect that matters and are only useful if you dedicate an excessive amount of effort into making them useful when it'd be easier and more efficient to use something better
some mons might be bad in the campaign (either by becoming obsolete very quickly or just bad from the start, or because by the time they become useful there's almost nothing left to use them on) but good in competitive, and some mons might be good in the campaign but bad in competitive (usually due to being outclassed by better options or having some combination of flaws that are easy to mitigate against campaign AI but severe liabilities in PVP against thinking human players)
it all comes down to the context a mon is used in. like any game, Pokemon is made up of countless interconnected mechanics and interactions; how they act will inevitably depend on which of those mechanics are in play at any given moment, and how they interact.
in short:
if using it is way more effort than it's worth in the campaign or competitive play, then it's a shitmon
