"Liar": it's kind of a mean word. Nobody wants to be called a liar, after all. To be totally honest, though, Lyra is a liar. She exaggerates quite a lot and often straight-up doesn't tell the truth. The Golden Compass, however, takes a much larger view of what Lyra does with words. In some ways the novel portrays her as a persuasive and powerful artist. Her "lies and deception" turn into something positive by the end of the novel – a form of persuasion that can be put to good use. Along with the theme of lies and deceit comes its opposite: truth and wisdom. Isn't it interesting that the biggest "liar" in the book is given the alethiometer, a truth reader?
Though Lyra may seem to be a powerless child, her clever mind and silver tongue give her a great deal of power, which she uses to her advantage over many characters, including Iofur and Mrs. Coulter.
Over the course of the book, Lyra switches from using lies for bad purposes (gaining attention, bullying other kids) to using lies for good purposes.
Lyra lies far more than she should. She could get out of most sticky situations in the book with out being deceitful.