It's a potential ad hominem and the context defines whether it is an insult or not. Unfortunately many people don't seem to grasp that context matters, there are plenty of words that can be used as an insult that are not otherwise defined as insulting.
Black, white and yellow are colours that are good example of innocuous words that can be used as insults. Even "girl" is still used as an insult.

Generally "ok boomer" is an intentional insult. It the same as stating anything the "boomer demographic" says or thinks is worthless, it's dismissive and rude. Ironically this generally comes from a demographic that typically are not listened to and are told to be quiet because they allegedly know nothing. It's quite hypocritical, that knife cuts both ways and everyone should stop being toxic.