No, it is not possible to word wrap the field alias text in a pop-up in ArcGIS Online. As a workaround, open the hosted feature layer in ArcGIS Pro, configure the pop-up, and overwrite the hosted feature layer to ArcGIS Online.
- Log in to ArcGIS Pro using ArcGIS Online login credentials and open the Catalog pane. Refer to ArcGIS Pro: Open the Catalog pane for instructions.
- In the Catalog pane, click the Portal tab and navigate to the hosted feature layer.
- Drag the hosted feature layer to the map.
- Configure the pop-ups to word wrap the field alias text in ArcGIS Pro. Refer to How To: Stack the field name in a pop-up in ArcGIS Pro for instructions.
The image below shows the field alias 'Bench Length In Inches' appears wrapped in ArcGIS Online Map Viewer.