The most likely reason people think Sticker Star and Mario Party 9 were mistakes because...just look at the context they were released.
Paper Mario: Sticker Star was the first handheld Paper Mario game that originally featured the Thousand-Year Door system people wanted after they had to deal with a changed formula in the form of Super Paper Mario. It's great to have a game that plays differently every so once in a while but people would also like a newer game in the form of the older style they were used to. And Paper Mario: Sticker Star looked exactly like that. It was basically the Paper Mario game they asked for. That is, until we get some drastic changes later in development which nixed the promising partner system and the battles and we got Paper Mario: Sticker Star. Of course, that's ok if the changes make sense and were still fun to play, but it shattered people's expectations nevertheless. But the problem was that the new gameplay system
also didn't work out for everybody who were still hoping for a decent Paper Mario game. Gone were the older stuff they grew up with such as established characters or at least a memorable plot. Bowser and Luigi don't even speak any lines, and
they're one of the most talkative characters in the game. The sticker battle system was also flawed but as I said, I can't say HOW it's flawed because I personally haven't played it.
As for Mario Party 9, the gap between it and Mario Party DS was the longest in the Mario Party series. Then we got Mario Party 9, which people actually wanted to be just as good (actually better) than Mario Party 8. The problem is the car system, which was horribly flawed and luck-based, plus the removal of items which also helped played the game in your favor and are necessary to win Mario Party matches. Also, another large part of the Mario Party system, arguably the biggest and only reason to play Mario Party, the minigames, are now an after thought and are not required, as they are in previous Mario Party games, to win the match. It's a sad thing too: Mario Party 9 has arguably better minigames than Mario Party 8.
Luigidaisy1 said:
Nintendo is in fact, the place to go for quality games
That's not necessarily true. I'm a die-hard Mario fan but I still had great quality enjoyment with other games not associated with the Mario franchise, nor with Nintendo at all. I suggest you open up a bit and maybe try a recommended game that suits your tastes outside the Nintendo spectrum.