Ngrok creates a secure tunnel from one of Ngrok's web addresses (e.g. https://whatever.ngrok.com) to the developer's laptop. It's useful when developing software since we don't need to deploy code to external servers to test it with other people or third party providers. You can read more about it here: https://ngrok.com/

So in other words, no - ngrok alone can't harm you. Althought the developer could be running a malicious site on the other end of the tunnel. The risks are the same as with any other website.