Yes, it is possible to edit GraphML files with applications other than yEd. GraphML is an XML based format and thus may be edited in text editors, dedicated XML editors, and custom applications.
Moreover, yEd does offer ways to view additional information aside from its HTML export:
Both, nodes and edges offer a "Description" property. The contents of this property is displayed as tool tip text when the mouse hovers over a node or edge in yEd.
yEd offers the possibility of "visualizing" values of custom properties using its propertes mapper (see "Edit" -> "Properties Mapper"). The properties mapper can be used to e.g. set the value of a given property as label text for an element or copy the value of a given property to the corresponding element's "Description" ("Tooltip") property.
Finally, custom property values of selected elements are shown in the properties table in yEd's lower right corner.