Liara's cameo in the retail version of ME2 only feels jarring and OOC if you take it at face value, and fail to read between the lines and analyse her words and actions. If the player is able to do so, and they did not misinterpret her character as it appeared in ME1, then I think you're able to see it as a believeable development and progression of her character. I've always maintained that her character development required additional clarification and elaboration that the scene on Illium did not provide, and this was proven correct with LotSB. I'm pleased!
Regarding Liara's supposed "shyness," I'm always reluctant to use that term to describe Liara's personality as its connotations imply traits that are not applicable to her character. She is more than able to speak her mind and be assertive when necessary, and is not a submissive wallflower that defers to what is expected or demanded of her. I've always loved the scene in which she argues with Charles Saracino, and it's great to watch her denounce his jingoistic approach to politics.
I just think that, similar to Kaidan, she is an introverted person who enjoys solitude, and is also somewhat inexperienced with social interactions. This is a consequence of what I believe to be a relatively sheltered upbringing, and her decades spent alone on remote digsites. Social inexperience does not equate to meekness, though. She is more than able to defend herself in social situations, even to the extent of defying a Krogan Battlemaster when escaping Therum, in which she vows to never assist Saren.
I suppose this could also cause something of a vicious circle, as she does mention that her tendency to make a fool of herself, a result of her social inexperience, causes her preference for solitude.