If you want my opinion, the "monument" refers to the whole structure and is "Monument aux Forains…".
https://commons.wikimedia.org/wiki/File:Monumentauxforainssquaredelaviation.jpg

Pierrot statue is very prominent, but is only a part of this monument.  Some documents refer to it metonymically, that can explain the confusion.

I think Raphaël’s interpretation is correct.  Following the name on monument.heritage.brussels makes sense.

How about drawing an area for the monument + a node with memorial=statue for Pierrot?