The Vilna Gaon abolished the minhag of placing trees in shul on Shavuos so as not to appear even similar to the nochrim.
To the best of my knowledge that is the minhag in all of Klal Yisroel ad hayom hazeh. Indeed, in America there is no “sabba succa” nor any lights which resemble kratzmach lights in the succa of any yidden.
Nobody is saying it’s assur to look at them, but apparently there is something to it.