I get you, I just think the decision for it to behave more human-like is a bit of an arbitrary one. Looking at its historical Pokedex entries, it could have very well transformed into more of a stellar spacecraft designed to steal humans to where it came from. Or a more intricate Psychic weapon designed to control the PokéEarth's population, etc. While its Mega is somewhat consistent with its lore (barring the "supplanting humanity" aspect that was suddenly introduced in ZA, but fair enough), there are many other lore-consistent options they could have chosen from which would make it consistent in design quality with other Megas. I think that's the key.
Further, I think Starmie fans are just upset that statistically, it is super anomalous insofar as it is the Mega that changes the least, and by a large margin (with Sableye being second, then big jumps from there - probably Slowbro, Altaria, etc.), yet was one of the few Gen 1 Pokémon remaining who had received no love (forme, evolution, mega) since 1996.