How do I get related table attributes to appear all on the first page of a popup in an ArcGIS Online web map?
Yes, but you need to make sure that your original relationship class has a 1-to-1 relationship. If it's 1-to-1, then you can simply add the relationship fields to your pop-up. If it's 1-to-many, you only get the option to add the "summary statistics" for a field from the related table.
You can read about it at Configure pop-ups (Map Viewer Classic).