Is MongoDB index creation single threaded?
There is an OPEN issue already created in MongoDB JIRA for the same.

As per the issue, MongoDB index creation is not Multi-Threaded.

use multiple cores for index sort-phase

Multi-threaded index creation

UPDATE

Try increasing the maxIndexBuildMemoryUsageMegabytes parameter value.

The default value is 500 MB.

What does it do?

Limits the amount of memory that the simultaneous foreground index builds on one collection may consume for the duration of the builds.

So by increasing this limit, may increase the performance of index creation. For Example:

db.adminCommand( { setParameter: 1, maxIndexBuildMemoryUsageMegabytes: 70000 } )