Gosh yes, you can't call Yank derogatory. Look at the song your army and in WWII:
"The Yanks are a comin', the Yanks are a comin', a drum-rum-tumin everywhere!"
If you are going to yell out very loudly that you are Yanks and you are 'a-comin' then it is not surprising that the world calls you that...
Now... where have all the nice girls gone? Kes? Leia? Evy? Lily? Anyone?