I agree with Musk, and with Trump (to a degree).
From a realistic standpoint, China abuses their quasi-free-market economy status to subsidize their products to undercut foreign competitors. Pretty easy to do when you pay your workers $1/h. This is well-established, and is not open for debate (even the far far far left will admit this - and it can be confirmed with multiple sources).
For 20 years, previous administrations have tried to "play nice" and find ways to motivate China to not dump products at or below their already reduced "cost". This policy has flat out failed to yield results, and it's disappointing that it has been tried for so long with negative results.
Additionally, as Musk points out, China tariffs imports from the US heavily, while we do not do the same (his example was autos - they tariff our cars 25%, we tariff theirs 2.5% - but this applies to a whole host of products, not just cars).
I can respect the position the administration has taken, as a businessman. If you have bent over backwards and don't see anything being reciprocated, it's time to take another approach. In this case, we have SIGNIFICANT leverage with China because we buy so much @#$% from them. Much much more than we sell to them.
The tariffs, in a nutshell, are completely fair, and needed to send a wake-up call to foreign powers that we won't just roll over anymore on unfair trade practices.