The example password "Tammy's#18bdayBIGpartyDroppedshibesweaterinpoolGMasonUsoon:(3yearsislong" on that page is weak.

Generating a NXT password is basically the same problem as creating a Bitcoin private key. bitaddress.org is to my knowledge a fairly trusted site. You can take the private key it generates and use it as your password.

The human brain should not be used to generate a password because it is not a good source of randomness.