This is a standalone npm binary to convert json-cov to html-cov for mocha, straight forward repackaged code from this thread, as pointed out by lazd.
npm install -g json2htmlcov
$cat cov.json | json2htmlcov > cov.html
There is another useful json-cov to cobertura xml format tool: mocha-cobertura-reporter, also using jade to render xml from json-cov.