You should not expect newspaper headlines and the titles of magazine articles to conform to traditional prescriptive grammar rules.
When you read that short title, you could make it into a longer/fuller sentence if you wish, eg:
The Tesla car design and manufacturing company has announced plans to produce mainstream electric cars.
The Tesla car design and manufacturing company has announced plans to produce electric cars suitable for mainstream car buyers.
We agree on the meaning. Let me comment on the grammar.
The article below the headline uses the word "mainstream" 3 times: twice as a noun ("bringing them into the mainstream") and once as an adjective ("become mainstream"). The phrase "take something mainstream" is used only in the headline. And headlines and titles have a less restrictive "grammar", as L points out.
However, I have read that phrase enough times ("take something mainstream") that I suspect it will soon be considered common usage (and therefore correct). Maybe it is already. Maybe it will be in a couple years.
At that time, grammarians can decide if it is ellipsis (omitting "into the") or a new adverb (which dictionaries will add).