Does a Mining World designation improve strategic resource synthesis?
No, the mining world designation increased the output of miners and strategic resource gatherers (that is, mote harvesters, crystal miners and gas extractors).
If you want to increase the output of the jobs that transform minerals in strategic resources (chemists, translucers and gas refiners) you need to select refinery world as designation for that planet.
https://stellaris.paradoxwikis.com/Designation