Do you see the pulpit as being a space that's particularly "set apart" from the rest of the church? If a clown were to give the message but not from the pulpit would that make a difference?
I'm not sure that "watering down" your hypothetical event, feature after feature, is worth pursuing much further. Having the clown show up only once rather than quarterly was one change, and now it's that the clown might not be using the pulpit after all.
Anyway, what was described did seem irreverent and could understandably cause some seekers or visitors to think twice about choosing such a church as this one for their own. That's the basic answer.
This clown event would logically raise questions about what other kinds of gimmicks that church finds acceptable. None of this is on the level of denying a cardinal doctrine of the Christian faith, you know, but it can matter when it comes to the scenario we were asked about.
But yes, many churches do consider the pulpit to be a special item normally reserved for a special purpose. I am reminded of the time I was part of a civic meeting which for purposes of accommodating the size of the audience was being held in a Baptist church, and the pastor asked each presenter/speaker to go to another part of the front of the church when making his or her remarks rather than use the pulpit for the purpose. And that was a
Baptist church.
If we were to make a comparison, would you think that using the church's altar for a buffet table on the occasion of a parish dinner would be a good idea? Why not? Congregational meetings are a good thing, right? Maybe that church's social hall was temporarily unavailable because it was being remodeled, etc., etc.