No, it is not. The world would be a much better place if we all started doing what is right. This isn't difficult: stealing and copying API design and API documentation is wrong.
Make a note of the company, people who work there, and VCs who invested. Computers are good at quickly finding information: one day you might want to do business with one of those people or VCs and digging up this information might change your mind.
Copying APIs is fine, as far as I'm concerned.
The world is as better place if hardware and software is compatible with each other, and common interfaces enable that.
But I can spot you that anyhow, because there's a perfectly reasonable fallback position: This is illegal. It's a copyright infringement at the very least, and possibly other things as well. And being "illegal", the "what are you gonna do about it" actually has clear, well defined answers. Smartcar.com is doing them a favor by serving a cease & desist, as there is nothing preventing them from moving straight to legal action.
That you wish something was true doesn't make it so, unfortunately.
What is not OK is copying the documentation verbatim, that is copyrighted. However, that is, frankly, the least of the problems (and easiest to rectify) when someone is taking your product idea wholesale.
Copying text or pictures would be copyright infringement.
https://slatestarcodex.com/2014/07/30/meditations-on-moloch/
That doesn't mean we ought to just give up and worship whatever worst thing capitalism produces. If you look at Ginsberg's long history of political activism as an example it's certainly not what he and those around him lived.
At a separate scale from that discussion we need to "play our side" of the game, and that will necessarily involve your personal interpretation of ethics. I believe it is a kind of laziness to dismiss that.
And I would happily live in a poorer society with better ethics any day (given some minimum threshold of live quality).
So inferior goods (using the economic term) can add low-cost options for consumers and are thus a social benefit.
While copying itself isn't bad, in practice those companies often releasing products that are unfit for sale or engage in deceptive advertising.
I hope most here can agree that taking a web page, doing command-C, command-V, changing a few words in a small amount and passing it off as your own work is wrong.
This is not even about cloning an API.