If he had used the image of a Christian monk, that would have been fine with the vast majority of people. In general, a cartoon impression of a monk is not considered insulting, nor is it considered bad taste[1]. The image is used to associate the service with all that is considered 'good' about monks. As this reminds people that monks are generally seen as 'good' people, this in turn benefits the view people have of the religion in question.

If the net effect of him using this image is that people are favourably reminded of Buddhism, isn't that a good thing?

[1] Consider the number of hotels, restaurants, cafes and bars named after monks (/friars/priests/popes/apostles/...)