Yes I noticed this too when I did a clean install of Sierra GM last week. Annoyed the * out of me.
However, I noticed when you create a new account once Sierra is installed, you can start it's name with a capital letter.
Really strange, no? So I wiped my drive once more, and set up Sierra with a temporary account first (and didn't log in with my Apple ID there). Then, from that account, I set up my regular account, of which the name starts with a capital letter.
OCD much?