How to get mean coordinates of features from a point layer?

Centroid property is only for polygon layers. If you have a point layer you can calculate the "Mean Coordinates" from Qgis geoalgorithms.

If your points have a common attribute you can run "Vector > Geoprocessing Tools > Convex Hull(s)..."

Input Vector = points
Create convex hulls based on input field = check, use the field with the
                                           common attribute

This will create a polygon for each group of points. Now you can create your generalised points "Vector > Geometry Tools > Polygon Centroids..."

Convex hull creates a minimum area polygon around the points. The centroid of which will then be the generalized centroid for each point cluster.