No, the PDF is being digitally (cryptographically) signed but it also overstamps the PDF document with the annotation shown above as a visible hint that the document has been digitally signed. @antonis just wants to have the functionality to customise the appearance of the visible hint, which functionality does not exist.
The visible hint is a bit misleading because I could easily sign a document as Irvine Wade but make the visible hint say Antonios … and the document would pass cryptographic validation. So the user really needs to look at the Document Properties and see what the actual Subject Common Name on the certificate is and confirm that it matches the visible hint, if the visible hint is present - or alternatively take the opportunity to open the signatures panel when offered to do so on opening a digitally signed document.
There would be some argument for giving the user the option of turning off the visible hint altogether.
Sure. It could work differently. You’ve built from source so you could easily completely override the text that is included so that it does not come from the certificate but instead is your name hardcoded in title case, or you could override it to convert the name on the certificate into title case before using it. (The rules for title case are locale / language sensitive though. Maybe you only need Greek to work.)