Might malignant melanomas cause weight loss in cancer patients?

If large can consume. Melanomas, like any cancer, will consume calories at the expense of healthy tissue in addition to suppressing appetite.
Melanoma. In general, the body's response to wide spread cancer like metastatic melanoma is to produce chemical signals that induce 'cachexia' - a 'wasting' state of health that involves tissue breakdown rather than repair and growth. This is a late-stage finding in cancer patients.