I need to create a single line text using Inkscape. So I write the text:

then I select this Object, I select extension, text hershey and I selext EMSAllure, then I click ok. The result is this:

As you can see there is a mono line trace

but the lines are all jagged. Is there a way to make the path have smooth corners automatically?

Use different less jagged font in hershey text, use path simplify command or apply bspline LPE.