Why do so many counterpart pokemon (frillish/jellicent, basculin, shellos/gastrodon, latios & latias, sawk & throh) seem to divide into blue and red specifically? (Also, did people model gendered fashion off of jellicent, or did they get bred so males had mustaches and females had lips and eyelashes?)

Coincidence.

That may sound like I’m being facetious, but there have been a number of studies on this point, all of which came to this exact conclusion.

The same could be said in response to your question concerning jellicent. Jellicent simply evolved to look like that on their own, and human concepts of gendered fashion just happened to mirror the jellicent through a completely unconnected (to the jellicent, that is) series of cultural shifts. It’s quite fascinating, really.

Frillish and Jellicent

Frillish
The Floating Pokémon
Type: Water/Ghost
Official Registration #: 592
Entry: This mysterious, jellyfish-like pokémon floats close to the surface of warm seas. When in close proximity to its prey—which consists of anything with a large enough reservoir of life force, humans included and especially—they reach up with their veil-like tentacles, paralyze their target with poison, and drag their stunned victims into the depths of the ocean. For this reason, in Unova, this pokémon is feared as a highly dangerous predator. However, in Johto, where local stories involve seabirds that generate hurricanes with a slight flap of their wings, this pokémon is prized as something that’s highly delicious when brined, dried, and served with vegetables and a little bit of vinegar.

Jellicent
The Floating Pokémon
Type: Water/Ghost
Official Registration #: 593
Entry: The evolved form of frillish, by battle experience. It is said that ships that wander into jellicent territory are doomed to be sunk, with all hands on deck lost. This is true in Unova, but ships from more eastern regions have been far luckier. Research has gone into why this might be, but the author already has a pretty solid theory: misplaced retribution over the fact that some of us brine, dry, and serve their preevolved forms with vegetables and a little bit of vinegar.