The moment she says she can't be that callous against her own people, I wanted to tell Steve to head back to the Normandy to get another squadmate and leave the asari on the ship. She also doesn't do nothing when the ponytail runs towards her.
I wouldn't say she is the reason the mission failed. I would blame her pathetic species for that. Had they revealed the artifact earlier, the encounter with Kai Leng may never of happened or at least the result might be different
In the Codex entry for Thessia, there is a brief bit of information as for the way Thessia's government is run, and how its fractured state in politics long before the Reaper War, coupled with endless deliberations among the Matriarchs, is why Thessia and the asari were so ill prepared for the War. I always felt that was a missed opportunity. It felt like a slightly realistic way for a government of republics to handle such a situation as a war for the Reapers. Not all races were as prepared and ready to go to action like the turians or salarians (to varying degrees of success), some were just paralysed with indecision, which rings true as has been seen in history, though I am not comparing the Reaper War to any example in history, just that indecision in a crisis isn't unique to the asari.
So yes, Liara should not be blamed for the failure of the mission. Nor was it Councillor Tevos or Irissa. It was Thessia's government, unable to unite and come to a cohesive decision among the republics, instead the Matriarchs centuries of wisdom and preference for taking time to deal with a situation made them ill suited and ill prepared politically for such a conflict. Which is sad, considering the asari commando units and there military being capable of handling it to an equal extent as the turians were doing, had they prepared for the Reapers arrival on Thessia.
Kai Leng was also supposed to be better than your squadmates and on par with Shepard. It wasn't well executed, but he kills Thane and demolishes Miranda without Shepard's help.
Kai Leng was so terribly inconsistent. Books and comics tried to arguable degrees of success to establish Kai Leng as this skilled threat that everyone should be afraid of and to be on guard. We see this with Miranda, but the rest of the time, Kai Leng is a cowardly, sword waving loon who just can't make his mind up what he wants to be. An Assassin, a psycho who likes killing salarian councillor fun time? Oh, now he's a swordsman? No, he just runs away. Now he takes out my car, but not me? Then he just retreats into a lift with Phantoms.
Then there's Thessia. Sheild recharging bullcrap. But that time, we should have just wiped the floor with the pushover. But no. Suddenly Kai Leng is a badass, coupled with Liara's sudden immobility, and Kai Leng is suddenly the skilled threat BioWare were trying to make us afaird of before but failed.
Horizon he just there, to kick Miranda's ass, which again is bull given what we've seen of him so far. I'd like to believe Miranda would have kicked his stupid ass, but no Kai Leng is made a badass again for reasons, and Miranda is a another character sucked into Kai Leng's vortex of damaging and bringing down the quality of everything he touches. Thane suffered. Liara suffered. Miranda suffered.