I used to love toppers back in the day when I first started collecting, and would make my own. But I only thought they were cool if they were wired into the game and interactive somehow. Over the years I've completely outgrown toppers, but the worst by far are when people put a bunch of toys, particularly stuffed animals, on top of the game. Tacky.
As for in-game mods, I really can't stand the spiral wireform mod for TZ. So many TZ mods are unnecessary, but at least most of them are merely cosmetic and are replicating one of the items on the playfield/translite, which are from the shows. The spiral wireform is really forced and in my opinion worsens gameplay!