Many Jews who immigrated to the US in several major waves of immigration (mid-late 1800s all the way to post WWII) were from central and eastern Europe (Ashkenazi Jews, where "Ashkenaz" refers to Northern Europe/Germany), and spoke Yiddish in addition to German or Russian or Polish, or whichever other languages were spoken where they were from. Yiddish is a Germanic Jewish language, and as such when Jews took surnames (voluntarily or otherwise), they often took surnames in the languages that they spoke. So, you get Jewish surnames in all sorts of languages, but quite a few of them are either in German, or Yiddish, or became Germanized.
For example, my family immigrated to the US in the 1890s/1910s, from a Polish-speaking city in what's now Ukraine. I've got a copy of the Holocaust memorial book written up by the survivors in the US and Israel in the '60s, and at the end there's a register of the Jewish families present just before the Holocaust. If I look at the list of family names, there are plenty of Slavic names (Polak, Modnik, Sklar), Hebrew names (Cohen, Shor, Shachar, Ashkenazi), but by far the most are German or Yiddish (Eisenberg, Goldstein, Weisberg, Wasserman, Zonenshein, Lichtenfeld, Schmidt, etc).
Now, most of the German and Yiddish names are either locations or occupations (-berg for mountains, Schmidt for a smith, Glaser/Glazer for a glassworker). Very few of those are unique to Ashkenazi Jews, and are also common among Gentile Germans and German-speaking Europeans. So, where there are fewer Ashkenazi Jews (the Netherlands, for example), German-language surnames might be more stereotypically un-Jewish. However, in America, where Ashkenazi Jews are much more common, it's more likely that a German surname might be stereotypically associated with Jews in general.
Sources: personal knowledge of Yiddish and German, The Memorial Book of Czortkow, Ukraine