When “mom” is descriptive, it’s lowercase. When it’s used in place of a name, like a nickname, it’s capitalized.
Scribes writing with the early Roman alphabet didn’t have to choose between uppercase and lowercase letters because there were no lowercase letters—all the letters were what we think of today as capitals. Lowercase letters came much later, as did the names “uppercase” and “lowercase.”
Uppercase and Lowercase
In 1382, the Wycliffe Bible was the first written reference to mention “capital” letters, according to the Oxford English Dictionary. The terms “uppercase” and “lowercase” came later and reflected the way compositors arranged the boxes that held the individual letters used in printing. The larger letters were literally stored in an upper case, and the smaller letters were stored in a lower case (along with the type for punctuation and spaces).