Is JS an reasonable choice for a DevOps internal tool (i.e. cloud-formation runner, scripts)?
Definitely. Especially when using AWS services, the aws-sdk for JavaScript is superb.

Writing code using JavaScript in a shop that has both frontend people writing JS in some framework (Angular/React/Vue/...) and backend people using Node.js removes barriers. Just pushing to use JS provides the org. with even more developers who don't have barriers to reading and fixing infrastructure code.

For example, I am quite proficient in using Python, Ruby, Shell, JavaScript (even Perl). But when it comes to fixing or adding something to HashiCorp's tools like Terraform, I just don't have the time to learn Go sufficiently enough - so I don't do it. If a tool like Terraform was written using JavaScript, I would have added my contribution for that missing service a long time ago.

JavaScript support is excellent in AWS Lambda as well, and that is quite a big 
deal. For example, one of the ways to make CloudFormation do things you never thought it could is to write an AWS::CloudFormation::CustomResource implemented in a Lambda function.

Another good example is streaming CloudWatch Logs into ElasticSearch. If you ever try to press that button you discover that AWS just creates a JavaScript function for you in Lambda.

JavaScript is everywhere in DevOps and AWS, doing the same thing with Java, or C# or even Go would take much more time.

Should you bring a "DevOps Engineer" in the future (please don't), and that person does not know JavaScript while all your code is the backend and frontend is in JavaScript then simply don't hire such a person. It is a bad fit for both you and them.