When publishing a hosted feature service with labels from ArcMap to ArcGIS Online, the labels do not display in the web map.
This is expected behavior. When publishing to ArcGIS Online, the labels do not appear in the web map because labels created in ArcMap are displayed differently than in ArcGIS Online. Labels are part of the map document layer properties and do not carry over to ArcGIS Online as displayed by default in the feature service.
The following workarounds are possible solutions for this problem.
After adding the layer to a web map, create the labels for the features in ArcGIS Online by following the instructions in ArcGIS Online Help: Create labels.
Note: The Feature Outline Masks tool is only available with an ArcGIS Desktop Advanced license. For more information, refer to the licensing information for this tool.
In ArcMap, add the labels to the map and publish the map as a tile service to ArcGIS Online. For more detailed steps, refer to ArcGIS Online Help: Define and publish a map in ArcMap.
By publishing the map as a tile service, the labels added in ArcMap are published and displayed with the data. Once published, however, the labels and data cannot be edited. This is because the tile service consists of tiled images of the map and does not display the label or feature geometry.
Note: Publishing a tiled map service consumes credits. For more information, refer to ArcGIS Web Help: Understand credits.