Hello,

I have just learned that all NetBSD mirror sites are not encrypted. Can anyone tell me if is safe to install NetBSD and its packages when connecting to those sites?

I am not really sure what you want to be encrypted. You would verify downloads by the published checksums of the files, having them encrypted during download does not really get you any added safety in that respect.

That said, it does not appear to be true that all of their mirror sites are not encrypted.
The website and download mirror link I followed are availabile encrypted, here for example:

NetBSD Home HTTPS

NetBSD HTTPS Download Mirror

NetBSD Download mirrors HTTPS

If you follow a mirror link the instructions are there to download via anoncvs over SSH - secure.

Perhaps you could explain a little more what you mean by "not encrypted", and what you would expect instead.