When Julia states herself as "not clever", what evidence backs this statement up? Part 2 chapter 3.
Interestingly, I think although Julia says she is "not clever", her actions in a number of different ways show completely the opposite. It is clear that she has been very successful so far in dodging party attention - it is she who sets the rendezvous sites and also suggests to Winston that they engage in party activities to avoid attention from the party. In fact, it is Julia who guides Winston through the start of their illicit relationship - it is clear she displays significant intelligence in this respect.
You could also argue that she shows her intelligence because she, unlike Winston, grasps the truth behind the party and its attitude towards sexuality. She identifies that the repression of sex was important for the party to maintain control, but also that the deprivation could be converted into hysteria which could then be harnessed in events such as Hate Week.
Perhaps the only way in which she could be said to be "not clever" is in her continuance of her relationship with Winston Smith. Perhaps we can understand how, through a combination of "luck, cunning and boldness", Julia has managed to escape the attentions of the party with a series of one-off relationships, but pursuing a long-term relationship with Winston Smith was doomed to failure.