It’s not meant dark as in absence of light. Dark is actually the english name, the japanese for said type is the evil type.
Now how come is it weak to bug? I think the reasoning is that dark is many ways the opposite of psychic. Psychic’s weaknesses are based on fears of humans, which are bugs, ghosts and also evil(dark). Dark doesn’t care about ghosts(guess why…), however at the end of the day, all evil has a weakness and the reason for bug is very likely that despite being the opposite of psychic, they share similarities.
It is also a common trope in media that evil people often have an irrational fear of something that’s not actually threatening.
Now when it comes to bug, I do think it should resists both psychic and dark, for the simple reason that bugs absolutely don’t care about such complicated things of human nature and if they feel like biting you, they will do that.
Now what doesn’t make much sense is…
steel resisting psychic
water resisting ice, instead of being weak to it. Water freezes when exposed to low temperature, aquatic live dies or goes dormant, or swims away, much like birds fly away from the cold, which makes flying weak to ice.
ground not resisting fire
ice not resisting flying(how does wind hurt ice exactly, if anything it just makes it more unpleasant for everyone else) and water(no water with a source of warmth doesn’t count)
steel not being weak to electric
fairy resisting bug
bug not being weak to poison
and yeah, normal being weak to fighting as well. The original idea might have been that it’s weak due to weaklings losing to stronger opponents, but with how the normal type looks now, being mostly represented by mons that are based of real life animals, that doesn’t make much sense. Should be weak to poison tbh, considering pollution is way more threatening to those than meatheads beating them up…
All in all, I would change a lot of things.