It’s more a “join in if you want / ignore it if not interested” sort of thing. We booked an autumn cruise a couple years back and several months before the date we got a notice that it would be a Halloween cruise. We brought costumes and indulged, but plenty of people didn’t. I remember activities and some decorations specific to the theme (the tree was cute), but not many. My feeling is that it was neither a reason to avoid a cruise nor a reason to book one for us.
Think of it as a buffet with some added dishes that you can choose to eat, or not.