Every landlord cares what his apartment looks like both from the inside and the outside. After all, it’s their house and it’s their good name on the line. In this case, we have to take their side. Why would you want to put something so personal on the window? And if he couldn’t find a better place for it, we suggest he get curtains.
Maybe this was on purpose, as kind of a joke, or because he lost a bet. Either way, there’s no reason why the landlord should put up with it. It’s not their place to tell their tenants what to do (or not do) on their own time in the privacy of their home, but in this case, it might be disturbing the entire street. That’s when his personal business becomes the landlord’s business.