I always found Liara kinda boring. She doesn't get much of an arc in 1 compared to Wrex or Garrus, she has minimal actual story importance compared to Tali or the Virmire duo, she's not even mandatory for the fight with her mother. She kinda felt like she was just......there because they wanted a hot alien chick.
Then in 2, she's sidelined for the majority of the game, aside from Lair of the Shadow Broker, but she still feels like an entirely different character. So she had an arc......entirely off-screen. Yes, it's shown in the comics, but it's kinda jarring if you haven't read those. I'm a firm believer that supplementary media should be supplementary, not required.
And in 3, they go all in on pushing her as Shepard's best pal and obviously-intended love interest, but still don't really give her much of a character arc. They keep telling you how important she is, but rarely actually show it. I feel like her best moment in the entire trilogy is meeting her father, and that's entirely on Aethyta being a riot.