Yeah, what your meme said is strictly speaking correct, however a bit of nuance is in place. The crime was leaving Germany and going to the U.S. skipping military service in the process and Germany didn't accept that and revoked his German citizenship as a result.
That's not half as bad as I expected from looking at the picture. But there is irony. Grandpa Trump didn't wanna stay in the U.S. but had to because he was no longer welcome in Germany... "If you don't like it, you can leave", eh? Donald, dig up your grandpa and send him back to Germany. Grandpa Trump came during the goldrush, eh?
And now I also know we always pronounced Trump's name wrong.... the "u" is in German pronounced as "ou" as in "you". Would Donald Trump feel insulted if I pronounced his name the German way?
And er... thanks Germany... See what your decision to revoke a hard-working man's citizenship brought the world today :P