。
She is a very good liar.
Literal
She [topic-は] very skilled liar is.
上手なうそつき packs a backhanded compliment — 上手 normally praises a skill, but here applied to lying it implies skill at deception. うそつき is built from うそ ('lie') + the masu-stem of つく ('to attach, tell [a lie]') + a person-noun nominalization, giving 'one who tells lies.' Same shape produces other person-nouns: 嘘つき, 嘘吐き, etc.