They wrote something about it being the old codename for the cloudant query language on which they based it.
it’s definitely no related to MangoDB
It’s MongoDB inspired obviously, but at the time MongoDB asked IBM/Cloudant to not call it that. They settled on Mango. The Cloudant product then became “Cloudant Query”, so CouchDB easily inherited the Mango nickname. We like it :)
Mango is also the name of a popular MongoDB driver for Perl, most commonly used with Mojolicious. I'm not sure which came first.