It seems "Mary Sue" just means "A character I don't like gets attention"...
Having attention is one of the main hallmarks of a Mary Sue, but by no means is it the only one. Neither does someone liking or not liking make a Mary Sue.
Mary Sue's have the following traits, and Liara has them too
Lack of Personality - Liara's character can be summed up in a sentence: "She exists to be Shepard's canon soft-hearted bestie." She doesn't undergo any permanent evolution in thought or personality
Everyone loves her, except the jerks - Kaidan's nice to Liara, Ashley's nice to Liara (and she doesn't really care for aliens), Garrus is nice to her. The only people who aren't nice to her are Wreav and Javik, two who are called out for being dicks (and not just in general, specifically about her)
Incorruptable - Despite having all the knowledge of the Shadow Broker, she's adamant about it "not falling into evil hands" and she never sullies herself with all the dirt she has.
The Shilling - Dear God, the shilling. In the Genesis comic, Shepard was very normal about describing his squadmates (Ashley was brave, Kaidan was a good by-the-book guy), but Liara was "as gifted as she was beautiful" and gave Shepard "a great fondness of the asari"
The Love(ish) attachment - Liara will always be Shepard's friend. No matter if you ignore her, no matter if you're a jerk to her, Shepard will still hug her.
Spotlight Stealing Squad - Dear God, there's really no need for her outside of one element, and she's still constantly around, getting DLC after DLC. You can never get rid of her.
The "bad past stigma" - The pureblood aspect, Liara whines about it in the first game, and she thinks it seems to make her unattractive or unlikable. And yet, Shepard doesn't seem to mind.
Unexplained badass - Liara was a wimpy scientist during in ME1 with only light gun training and average asari biotics. Okay, that's fine. Somehow, she trapsizes all around the galaxy and rescues Shepard's body
There's more, but this will start it.