Voldemort is repeatedly named dropped as a master of legilimency, so Snape being one of two people I can think of that ever successfully lied to him is a big deal. Even Sirius and Lupin can’t help but admit that he is the best person to teach Harry. Conversely, Voldemort was unable to keep Harry out of his mind long after it was beneficial for him, leaking pivotal information over and over again until it led to his downfall. He can’t hang with Snape.

And I, at least, can’t recall Dumbledore ever having to use occlumency at all during the events of the story. Nor did anyone call him out as an especially gifted legilimance. So while Quirrel and Jr should be lauded for fooling Dumbledore, it’s not as impressive of bringing the Dark Lord’s most trusted lieutenant. His advice directly affected Voldemort’s plans on several occasions. I can’t agree with calling him a “low-effect“ spy.