Is it frowned upon to have a figure in the introduction of a Masters thesis?
If a graphic clearly gets across the message then why not use it? Just check what the regulations specify though,

To elaborate a bit more you need to think about what the introduction is for. If it is  intended as an executive summary then a diagram that summarises the work could be very effective. If however it is setting the context of the work then I find it difficult to see how a diagram would help.