It's interesting, though, badges in bootstrap (the version currently used by XWiki) have their own border-radius, the @badge-border-radius , while labels have a fixed radius of 0.25em.

I guess the best here would be to use the @badge-border-radius.

As for paddings of these items:

• labels have paddings based on the font-size, in ems
• badges have fixed, hardcoded paddings.

However, as far as i can understand from documentation, labels are more geared towards being used inline than badges, so I guess that would be a strategy that would make sense in the case of mentions as well, as they're made to be used inline.