I think the bigger mistake is to think even if it was a good sample it could gauge people’s mood - incentives for posting to social media are either happy or outrage. You will never get a silent majority “just okay” or “neutral-low” by averaging between ecstatic and outraged.

"The squeaky wheel gets the grease"

I think the fact that two of the top ~10 happiest days in 2018 were attributed to a Korean pop star's birthday is excellent evidence that people of twitter are not, in fact, good samples of the people of the US.

Also, the sadness when Trump was elected despite relatively even vote totals tells you that Twitter skews to the left.