Unfortunately, yEd does not offer the option to turn text into outlines when exporting to SVG.
With the current version, the only workaround I can think of is using an SVG icon in the label instead of real text. I.e. create a SVG document that displays nothing but the desired text, select the label in yEd that should display said text, import the aforementioned SVG document as the label's icon, and remove the label's "normal" text.