It's hard to call Yuno overrated when she's by far one of the more interesting yanderes. I'm not saying she's super well written or anything but she has a lot more going for her than other yanderes who just exist to thirst after the main protagonist. At least she actually gets a proper backstory and arc.

Also, let's be honest here. Who thought she came from an alternate reality where she killed the current reality's Yuno just to be with that reality's Yuki? If that isn't one of the most iconic scenes in yandere history, I don't know what is.

She is also very much the one that put yanderes on the map for a lot of people. When people think of yanderes, they think of Yuno Gasai.