Your mentor role revolves around Council/Alliance relations. With all of them, really. But with Ash, it's more detailed stuff about humanity's destiny. And not just the buddy cop movie with Garrus. She also saw her whole squad die and you go through the same things with her and Garrus about that stuff. Secondly, I think the bomb placer at Virmire is supposed to die, and the one who goes with Kirrahe meant to live. That's just my personal take though. I think Ash changes from working with aliens like this. And develops her own thoughts and begins on her own path of leadership (which why she can part ways for awhile... and then becomes a Spectre herself).
Garrus is not a poor man's Shepard or learned his skills from him either. He's more of the Soldier-y aspect of an Infiltrator. He's both tech and marksman. He's a cop and learned how to shoot from his dad. He's also the same age apparently. So it's not exactly a mentor role. It's somewhat competitive. It's reflective of Turian/Human relations in general, and has nothing to do with bringing up a Soldier (specifically an Alliance Soldier). All we really mentor him about is morality, red tape, and justice.
Uhm... what?
What does Tali, Wrex or Liara or Garrus (i.e the aliens) have to do with the Alliance, in any of the arcs? What do any of them apart from Wrex have to do with the Council?
Tali is just on pilgrimage and can advise with the geth and help with Engineering
Wrex just tags along at first and his is the most direct greivance with the Council (but has nothing to do with the Alliance). Not that you take any steps to help with that, at least in the first game.
Garrus has nothing to do with either, he just wants to nail Saren and is conflicted between being a by the book cop or a renegade one
Liara is a Prothean expert only there to move the plot along and has a connection with one of the antagonists.
I honestly remember nothing about Kaidan. Something about headaches and killing his instructor? That inspires confidence...
Ashley's biggest chip is her family being blacklisted by the Alliance and her straightlaced religious and political views, which while you can discuss, she never really changes her opinion on. She accepts the aliens as allies and friends individually, but I don't recall a categorical reevaluation of her previous position on alien relations in the same vein as Pressley's. And where are you getting this humanity's destiny stuff? The was nothing like that in the game I played. The only help Shepard ever gives Ashley professionally is incidental. She's allowed to overcome her blacklisting by association. Shepard didn't recruit her she just sort of fell into his squad (literally, if I think about it). She holds her own and my Shepard at least, would help her out. But as it is there is zero mentorship available in the games for her.
And even if I grant that making Ashley more open minded and opening previously closed doors for her is active mentorship, that's like one example vs how many for Garrus? The latter is still the most actively groomed character on the squad.