Because democratic society requires us to agree on basic facts in order to function.
"People are wrong about" things like:
- who got more votes in the last US presidential election
- whether future elections in the US will be fair
- whether global warming is real
- whether Democratic politicians are literal lizard people who molest children
We have seen that when we can't agree on these basic facts, people take actions like
- attempting to overthrow the seat of government
- attempted assassination of political figures (or their spouses)
- election of politicians who say that they will overthrow elections that don't go in favor of their party
- enacting policies that threaten to create a climate catastrophe.
The reason that Musk's tweet at Hillary repeating the conspiracy theory that Paul Pelosi's assailant was a male prostitute is a problem is because, in providing an alternative (false) explanation for the attack, it contributes to hiding the truth on this issue. The truth being that we can draw a straight line from politicians' words to this attack.
And since we vote for those politicians, if people are wrong about this, then yeah, it does matter.