The term capitis diminutio has nothing to do with capital letters, the word capitis there is more closely related to how it is used in the term "capital punishment" i.e. it basically means "maximum." If a man in ancient Rome was punished with "capitis diminutio" it meant his status was diminished in the maximum way possible, i.e. by making him essentially a slave. It was not caused by writing your name with lower case letters, it was caused by doing heinous deeds.

Ancient Rome didn't even have a concept of capital letters and lower case letters, that didn't come about until printing presses existed (over a thousand years later) and they had two "cases" of letters to write pages with, an "upper case" which held a large, fancy font used for writing the initial letter on a page and a "lower case" which held a smaller, simpler font used for writing the main content of each page.