Inspirational material has been demonstrated to not help with happiness, self-esteem, or self-improvement over any significant amount of time. In fact it often has the opposite effect because of the letdown factor. The motivational industry has been pushing the stuff for decades, so we have decades worth of failures to prove the negative effects.
There is not nearly enough Demotivator material though. Demotivation helps you set your sights lower, so you avoid missing your goals and feeling bad about yourself. Plus, its funny instead of sad. I’d trade a hundred inspiration books for one t shirt.