For me, the worst thing about Sticker Star is that future games took inspiration from it.
Sure, Sticker Star itself was a poor game, but I'm also not necessarily against companies experimenting with their IPs as long as said IPs are in a decent place, and at that time Paper Mario wasn't really in a tough spot as I recall. The last Paper mario game we got was SPM, and while not as good as teh first 2 games I still thought it was overall decent.
After Sticker Star, had they went back to traditional PM, or atleast something closer to it, and made more exciting Paper mario games, I would find Paper Mario Sticker Star a bad but largely inoffensive game and just look back at it as "well, they tried something different and it simply didn't pan out. Oh well, **** happens."
Sadly though that's not how things went down.