I agree, completely- and I'll give further reasoning why.
When MM gave her statement to police after MS was originally arrested and charged, LB wasn't even on their radar. Only TB. She was likely only questioned about her knowledge of a truck theft and a murder relating to it. This could have likely been her only statement to police. Later on, as the investigation opened up to LB's disappearance, it's very plausible that she would miss news coverage of it. I can't even tell you the amount of people I know who have never even heard of DM, MS, TB, and certainly not LB, who was reported on disproportionately less than TB. And my friends are all quite progressive and educated and watch the news... It's easy to miss things.
Secondly, it's not unreasonable that she misunderstood the question. When she's asked "before this trial, had you ever heard of her?" maybe she misinterprets the word "trial". Maybe she means "before this investigation" or "before charges were laid in her death" or "before all this". On the contrary, maybe she didn't misunderstand the question, and the first time she's heard of LB was in fact, this trial. If you even meet with a crown attorney AT ALL to prepare, you only do so a day before or the morning of your testimony. I know this from first hand experience testifying in a trial.
And lastly, I'm just reluctant to call her a liar at all. You may all have your opinions about her, she may come across as uneducated, naive, young, aloof, etc. but liar just doesn't strike me. She has no real reason to lie about this. If she knew Laura; had met her or heard of her, she would have just said so. It doesn't implement her in LB's disappearance or murder in any way. And she's admitted to far worse things already. It's so incredibly plausible that their paths never crossed.