Is thymoma cancerous?

Sometimes. There are both malignant and nonmalignant formsmof thymoma.
See below. It depends on the grade of the thymoma; most are benign and after surgical resection do not recur but some do recur.
Yes. Thymoma is a type of cancer that begins in the thymus. Thymoma is generally a slow-growing tumor. The following links may provide more information: http://goo.Gl/mqtn1 and this one: http://goo.Gl/oetwz and this one: http://goo.Gl/yjnh5.