Should the message failed to signup because too many signups really be considered an error?

I agree. It's still an issue in 2.x-dev. We're getting a lot of those "errors" in our logs, which creates a lot of noise.

In this case exposing it as an error makes sense from the Mailchimp API perspective, but for the Drupal module it's not more than a warning and should be logged like this.

According to the Mailchimp API documentation https://mailchimp.com/developer/marketing/docs/errors/ all "Invalid Resource" on subscribe should be validation issues on the Mailchimp end. The MR here prevents logging those into the Drupal logs at all.