Yes, Germany received a million refugees from Syria last year alone, and ISIS has publicly announced their intent to "seed" the refugee flow with sleeper cells. Germany already had an extensive Turkish Muslim population that has been poorly integrated into German society, where second-generation immigrants are ripe targets for radicalization. As average citizens grow increasingly suspicious of Muslims, the ISIS recruitment may increase and continue that downward spiral of distrust, fanaticism and prejudice.
Germany has a terrorism problem that is quickly turning into a crisis. There have been numerous terrorists attacks in the name of the Islamic State. The country has accepted hundreds of thousands of Middle Eastern refugees; some of which are sympathetic to ISIS. There will unfortunately be more attacks in Germany, until the government gets serious about fighting terrorism.
As we in America have learned, most anti-terror measures end up being used against "normal" criminals without any ties to actual terror groups. Law enforcement loves to overcharge and then try to get a plea deal. This has nothing to do with terrorism, that's just the buzzword that everybody uses these days to try to make defendants lose their rights.
There is a terrorism problem in Germany, and it is worth fixing, but it is not a big problem. This girl is likely an isolated incident, as are most of the acts of terroism in Germany and around the world. She may not even be connected to the Islamic State.