The zerg would be difficult to assimilate though, they have such a short incubation period, and we know the Borg implants are added after birth. logistically it's complicated to manage.
I don't think they'd be worthy for assimilation as a whole either, as the underlings couldn't't meaningfully contribute to the Borg hive mind.
The Borg couldn't even assimilate 8472 aka the Undine. The Zerg make those bastards look like baby's first eugenics war. And thats not even factoring in Kerrigan's BS writing.