If you understand the Metroid series mythos. I don't think you could say Ridley is less important by Dark Samus. Dark Samus was basically killed off for good at the end of the Prime trilogy anyway.
Ridley is the leader of the Space Pirates, and has been in every Metroid game except three (II, Prime 2, and Hunters). By comparison, Dark Samus has only been in two Metroid games (Prime 2 and Prime 3), and maybe you could stretch it to three if you include her alterego from the first Prime.
Ridley also is the reason why Samus even became a bounty hunter in the first place (his attack on her colony is what orphaned her).
For those familiar with the Zelda series:
Ridley is to Ganondorf as Vaati is to Dark Samus. Dark Samus and Vaati both don't make any appearances outside of the storyline they are allocated to (the Prime/Phazon trilogy for Dark Samus, and the Four Swords trilogy for Vaati).
I sure hope you were referring to yourself on that.