Sybill is the great-great granddaughter of a genuine Seer, Cassandra Trelawney. Cassandra's gift has been much diluted over ensuing generations, although Sybill has inherited more than she knows. Half-believing in her own fibs about her talent (for she is at least ninety per cent fraud), Sybill has cultivated a dramatic manner and enjoys impressing her more gullible students with predictions of doom and disaster. She is gifted in the fortune teller's tricks; she accurately reads Neville's nervousness and suggestibility in his first class, and tells him he is about to break a cup, which he does. On other occasions, gullible students do her work for her. Professor Trelawney tells Lavender Brown that something she is dreading will happen to her on the sixteenth of October; when Lavender receives news on that day that her pet rabbit has died, she connects it instantly with the prediction. All of Hermione's logic and good sense (Lavender was not dreading the death of the rabbit, which was very young; the rabbit did not die on the sixteenth, but the previous day) are lost: Lavender wants to believe her unhappiness was foretold. By the law of averages, Professor Trelawney's rapid fire predictions sometimes hit the mark, but most of the time she is full of hot air and self-importance.
Nevertheless, Sybill does experience very rare flashes of genuine clairvoyance, which she can never remember afterwards. She secured her post at Hogwarts because she revealed, during her interview with Dumbledore, that she was the unconscious possessor of important knowledge. Dumbledore gave her sanctuary at the school, partly to protect her, partly in the hope that more genuine predictions would be forthcoming (he had to wait many years for the next).
-Pottermore (Emphasis mine, both italics and bold)
She knows more than she thinks. So yes, she should be given a little more credit (but not much).
