An adjective to replace "so random" (to describe one who is apt to say random things)

Solution 1:

Off the wall, informal, is defined by Google’s dictionary

1. eccentric or unconventional. synonyms: eccentric, zany, far out, freakish, quirky, idiosyncratic, unconventional, unorthodox, weird, outlandish, offbeat, off-center, bizarre, strange, unfamiliar; More 2. (of a person) angry. "the president was off the wall about the article"

“So random” is MIT/hacker slang from late 1960s and 1970s on into the 1990s.

Solution 2:

Idiosyncratic

1a: characterized by idiosyncrasy, e.g., a peculiarity of constitution or temperament : an individualizing characteristic or quality

2: characteristic peculiarity (as of temperament)

Others: quirky, silly, eccentric, batty, off her rocker

Solution 3:

Unpredictable, perhaps.

• likely to change suddenly and without reason and therefore not able to be predicted or depended on.

'Unpredictable' suggests an attribute that may be fun or may be irritating, which fits with describing someone as 'so random'.

Solution 4:

A more formal word for this would be

Aleatory

meaning "arising by chance", especially in the context of art or music, or art created with an element of randomness (arguably, random utterances is creating art using speech as a medium; for example, acting, or improv).

Jamie's latest stand-up comedy series garnered mixed reviews online; newer audiences raved over the cogent and biting social criticism woven into the jokes, while longtime fans were tired of predictable political jokes and preferred the comic's older, more aleatory style of toilet humour.

Solution 5:

Not the best alternative, but in the context of someone saying or doing something “random”, thus making them be “so random” in the moment, arbitrary might work.

Arbitrary

Based on random choice or personal whim, rather than any reason or system. ‘an arbitrary decision’