Why is Buffy making up a name for Sunnydale somehow worse than Spike calling it Sunnyhell when he also views it as home? Calling it Suckydale really doesn't strike me as having anything to do with not loving her mom anymore or not wanting to honor her memory, but that quite simply Sunnydale sucked. A lot. Just like how the high school sucked. A lot. Sure, there were good times (the Prom and a shiny umbrella) but it doesn't change the fact that it sucked. A lot.

I don't see why that's to read too much into what it means--I don't think it at all reflects on Buffy's love for Joyce.