No, the sign makes sense and highlights the main reason the Serbs are so opposed to the hotel. The proposed site was a historical building that was bombed by NATO during the war for Kosovo’s independence. Serbs pretty universally see those events as “American” bombings and often cite them as cause to despise the US. So, the sign is pointing out the insensitivity of an American developer building a hotel in that location by comparing it to a Japanese developer building a hotel in Pearl Harbor.
Given how deeply Serbia is within the sphere of Russian influence, I doubt they care one way or the other that it’s affiliated with Jared Kushner, they just don’t like that it would have been owned by an American.
And yet, that explains absolutely nothing about the relevance of Pearl Harbor to this situation, which is what this particular thread of the discussion was related to. Thanks for the downvote anyway, though.