If all cars are more expensive, and all manufacturers have lower margins Tesla will have lower margins too. For the stock price the competition won't be other car manufacturers but other industries.
It would be a "subsiding tide lowers all boats" kind of thing.
I've been thinking, when I younger and living in the UK, all sorts of things (including some with the word "tax" in them) were denounced by the local right wing as "stealth taxes".
Trump is talking about tariffs as if they're a tax on non-Americans, but they're paid for by Americans who import stuff, which is basically all Americans given where your oil, aluminium, and steel come from.
It would be a "subsiding tide lowers all boats" kind of thing.