As far as the comment that we cant show that clown masses are necessarily bad, if it does not give the priest an option in the GIRM, then yes it is bad. Priest are obligated to stick to the GIRM and not introduce anything themselves. Besides, the mass is where we go and give all our attention to Our Lord. We do not go there to bring attention to ourselves nor make some kind of statement.
I’m not a canon lawyer but Can 899 par 3, states “…. and all the faithful present, whether clergy or laity, participate together, in their own way, according to the diversity of orders and liturgical roles.” Clowns are not an order nor do they have any liturgical roles and in “their own way”, does not mean we can dress up in costumes either.