I’d really like to know the answer to this as well. Sometimes it
seems like a Pokémon’s stats are affected by their nature on here,
and other times it seems like they aren’t and the stats that
increase are just random.
Well... as far as I know, there are EVs (because of the existence
of Vitamins like Proteins, Carbos, etc. in Battle Shop) and Nature
that affect a Pokémon's stats on PokeHeroes.
As for IVs, perhaps there are IVs, but I assume that they aren't
shown, just like in core-series Pokémon games.
I don't think these stats really affect much unless you like
battling on PokeHeroes.