That would seem like the case, but it's not necessarily true since the first real "first mate" introduced in the series was Mohiji, and he was a heck of a lot weaker than Cabaji. What are the titles for Mohiji/Jango/Gin?
Mohji, Jango, and Gin are officially introduced as first mate. Jango is also the "acting captain."
Mohji wasn't necessarily weaker than Cabaji; Cabaji also went down with one hit (not counting the boot/hilt to the face he gets).
Gin was obviously stronger than Pearl, being able to shatter his shield.
As for the Kuroneko pirates, they don't have a second mate listed. Just the "ship guardians."