Yes, it would still be plain text , you do have to connect using the secure shell (SSH) protocol to the SSH port (22).
You can use putty just as a telnet client, but it would still be insecure, sorry I can see how I made that unclear.

If you actualy tried to connect using the secure shell protcol but set the port to 23 (telnet) not 22 (ssh) I think it would probably fail to connect.