What makes Thessia the emotional low point for Shepard isn't the loss of the planet itself, though losing the anchor of the galactic economy is certainly militarily significant. Its that Kai Leng defeating Shepard potentially foils the entire Crucible project, and by extension, the war effort. Shepard isn't depressed because he cares more about the Asari than anyone else, he's depressed because Thessia is potentially the point where the war is lost.
I only wish, but no, Shepard is upset because of Thessia. Shepard says, quote, "Thessia fell, and that's on me."
Which is either enormously hypocritical or darkly hilarious depending on what Shepard did before then.
Shepard: "In case you haven't noticed, Joker, we just lost a few million people. This isn't the time!"
*sacrifices Kahje to save Jondam Bau, "the hanar did this to themselves - I need all the spectres I can get"*
*kills the Rachni Queen*
*shoots Mordin, lies to Wrex about it, shoots Wrex, lies to C-Sec about why Wrex attacked*
*arranges for 17 million people to be slaughtered by robots, chooses "they were stupid" dialogue option*
*alternately, shoots Legion three times and denies that synthetics are alive to begin with*
Joker: "Uh... really, Commander? You want to go there?"
The only genocide in the game that Shepard
doesn't instigate is the one he gets upset about and takes responsibility for. He can be anything from callous to outright cowardly (lying to Wrex's face) about what he did to the others, but when it comes to the Asari, we can only choose varying degrees of deep sadness for them.
I think this is a legitimate criticism.
Part of the problem I think is that it is never exactly clear how the balance of power between the Citadel Council and local governments works. We do know that at least in some cases local governments enjoy a great deal of power and that the Council's authority isn't absolute. If Shepard cures the genophage for instance, its a Salarian dalatrass that is undermining the war effort rather than the Salarian Councilor, for example. But beyond that the relationship between local governments and the Council is a bit vague. So its hard to say how much of a role Tevos may or may not have had in concealing the Prothean V.I.
I think that exchange with Tevos (or her replacement) may have played out better had it been revealed that it was a local government that concealed the V.I., and that Tevos had been kept just as much in the dark as Shepard. Having the Councilor be someone who is attempting to do the right thing while being stonewalled by local politicians, would have made her more sympathetic.
The fact that traffic for the human embassy gets routed to the Asari embassy in the wake of the coup tells me there wasn't much thought put into how that was structured.