Just to note
She said to curb it, not stop it.
From a general social stability aspect, controlling immigration (refuge, guest worker, or permanent) is important. The numbers per year that a society can absorb is different every year, dependent on the economy, the society as a whole and of course regional issues. Each society is also different. Canada right now can take a lot of immigrants, given our ok economy and the already high number of immigrants and 2nd generation Canadians. During the 80's and 90's I remember alot more anti immigrant Canadians than I do now. For non Canadians, it is in the 70's that the immigration pattern for Canada changed dramatically from being heavily European, to more Asian primarily.
Germany with very little history of immigration (and what was done in the past not done very well by the German state (see Turkish immigration if I recall in the 50-60's). So accepting a 1.5 million refuge's in just two years is going to cause issues. Just to note, proportionally only in 2016 did Germany accept more immigrants than Canada does on a general annual basis.
So yes Hillary is right in this regard. If the goal of a country is to accept immigrants, it has to do so in a controlled manner as to ensure social stability, which will ensure that immigration in the future can continue.
Now as to the immigration issue/crisis in Europe specifically. The highest number of immigrants to the EU in the last few years were from countries destabilized by the EU and US (Libya, Syria, Iraq, along with various African and other Asian countries. Countries around Syria have take in far more refugees than Europe did (vastly more than the US did), despite the refugee's being a direct result of Nato/US actions in those countries. Jordan and Lebanon each have 1.5 million Syrian refugees, both countries are vastly smaller in size, population and economy than the EU or US. So I do not have much sympathy for the EU/Nato countries in seeing refugee's coming to their countries in large part due to their actions