The reason Bug-type could be seen as super-effective to Dark and neutral to Fighting is because many of the tokusatsu genre heroes were inspired by bugs in their outfits and designs. Ledian, Accelgor, Ninjask, Heracross have a certain hero/kamen vibe to them for example that stands out.

Huh, never had a clue about this until recently. The more I find out about Bug-type the more I appreciate it. It objectively has the best designs and definitely embodies the “monster” part of the franchise well. This also explains why the anime used Accelgor as a Kamen Rider’s partner Pokémon.