The display in Okular is independent from Kleopatra, so dropping it in Kleopatra should be fine.
If a QES certificate is available, Okular should highlight and add a filter for them (which is currently not working, see T6632: Okular: Highlight / preselect "nonRepudiation" certificates for qualified signatures)

I currently have a slight preference to drop bold and go with normal font. Werner would be ok with that, too.

@svuorela said, QES certs shouldn't be required to be on a smartcard.

Using an icon for QES certificates isn't that easy because we use an icon for smartcard certificates and any list item can have at most one icon. Moreover, QES certificates are very like stored on a smartcard (isn't that even a requirement?), i.e. an icon clash is basically guaranteed.

In T6632: Okular: Highlight / preselect "nonRepudiation" certificates for qualified signatures I had the impression, that some hint is useful for signing operations. Probably not so much in general.

Highlighting QES is mostly useful for Okular, I guess.
Maybe use a symbol with a pen? That should be self-explanatory.