At least in Texas, there is a good chance Musk's backing increased Democrats' margins.
I don't think Musk was even relevant in that it being Musk specifically.
In the post election analyses only a few stories are looking at how it was done. People didnt simply go to the polls and blindly vote Democrat. Look at the candidates and the platform they ran on, they all sidestepped culture war distractions, they didnt lean into divisiveness or the childish name calling -- no one wants to listen to that shit anymore. Even the victory speeches in Texas were about Texas and working for Texans. Same for the guy in Kansa and the ones in Iowa. They got back to real interaction with real people in the real world.
Oh and ICE killing a US citizen in south texas and covering it up for a year, then killing a Mexican citizen (who was in the process of immigrating) and then wildly lying about it -- among other things like raiding US citizen small businesses because the owners look Mexican -- doesn't win a lot of votes for the party doing it. They woke up the non-voting US citizen latino/hispanic population in Texas and other places.
He wasn't. But to the extent he had an effect, I think it was–net–to turn out Democrats and left-leaning independents.
A pro-Putin, pro-Trump racist, anti-Semitic billionaire building unpermited data centers and who killed children through DOGE and is constitutionally incapable of flying under the radar when he needs to is almost out of a political textbook for who you want in your opposition's camp.