I'm going to list the main reasons that I disliked it, so this may get long.
1. It had little story. It was literally bowser causes havoc, breaks the sticker star, and kidnaps peach. Other games had more interesting stories (Bowser steals the Star rod so that only his wishes are granted, The X-Nauts are trying to revive a 1,000 year old demon with the crystal stars, Count Bleck wants to destroy the entire universe)
2. It had no memorable characters. The other games had the partners/ pixls who were very memeorable, and even the NPCs were memorable (Don Pianta, Merlon, the previous 3: bowser) but this had Kirsti, who was as flat as the world that you inhabit.
3. There were no reason to battle enemies. They don't give any EXP, only coins which you use to buy more stickers. And because your attacks are stickers, which you consume, you eventually avoid battles to save enough stickers for the boss, speaking of which
4. The boss fights sucked. You could only defeat them efficiently if you use their Weakness 'Thing,' but the game gives no indication of what it's weakness is. As a reference, in TTYD, early on in hooktail castle, Kolorado's father writes in his letter that Hooktail dislikes the sound of crickets, and right after you get the ability to turn sideways, there is a badge that makes Mario's attacks sound like Crickets. However, if you somehow missed the badge, you can still beat him as the badge only affects his attack/defense. In this game, If you didn't find the specific Thing laying on the ground a few stages before the boss, then you'll waste all of your stickers trying to fight him. And if you win after using all your stickers, Kersti will ask you if there was a more efficient way of going about it. If a game basically demands that you use a guide to make sure you have the boss's weakness, then that isn't good. (and Bowser has 5 forms, each with their own weakness)