I agree totally about the toy collections or misc nick-nacks people put up there. I also agree with the various light up "signs" people put up there. The Laserific stuff, in some cases looks OK, but I don't get adding them to the game.
All that said, if a topper is wired to the game and tied into certain features, I think they can be cool if done well. I made a Stargate topper years ago with a swirling blue sphere inside of a custom painted stargate ring. Each of the chevrons on the ring had an LED in it tied to the ones on the playfield, so it was swirling with the "event horizon" and the chevrons lit to indicate playfield status. It was cool (despite the fact that TECHNICALLY the stargate wouldn't engage until after all the chevrons were lit, but...).