Well, it’s messy.
- A bio (sorry: credential) is understood as a separate component of meaning. That is normally taken as equivalent to a separate sentence, and hence should start with a capital.
- The bio (sorry: credential) is displayed inline with your name in answers. In fact, it is currently separated by a comma, which means that in that context it is not treated as a separate sentence.
- Credentials are displayed in menus. The usual practice for options in menus is for each option to be understood as a separate component of meaning—which, again, means capitalisation as a sentence.
- They aren’t real sentences, displayed in the context of continuous prose. For that reason, people aren’t going to be fussed about capitalisation, and the default behaviour when in doubt is to go for the least marked behaviour. Uncapitalised is the least marked behaviour.
- I don’t want my bio to look like it’s been txted by someone who ignores the shift key. ;-P
- Sorry. Credential.