Unfortunately, yEd is not able to automatically generate descriptive tool tip texts for imported symbols.
However, you do not need to edit any GraphML files for specifying tool tip texts manually. After importing new symbols, right-click the corresponding palette section to open its context menu and choose "Convert to Document". In the new document, select the symbol for which you want to specify a tool tip text and enter the desired text as "Palette ToolTip" in section "Data" in the properties view in yEd's lower right corner. Once done with the texts, open the palette section's context menu once more and choose "Adopt from Document".
By the way, your suggestion of using the id of the first g element without display="none" would result in the tool tip text "Layer_1" for 141 out of the 166 SVG symbols in AWS_Simple_Icons_svg_eps.zip. Personally, I do not see any benefit in lots and lots of "Layer_1" descriptions.