What could cause a tumor-like growth on a bearded dragon?

Without more detail - it could be anything from a harmless polyp to a cancerous tumour. Either way - a forum like this is no place to ask veterinary advice, as there's not way of knowing whether the contributor is qualified to advise you.

The best thing to do - would be to make an appointment with your vet. They'll examine the lump, and perhaps take a biopsy to test whether it's malignant or benign.