I don't think Elisabeth had a problem with Astrid in particular, but it is true that she preferred Leopold consider a Catholic princess. She also urged him to consider the Orthodox princesses of Romania (probably Princess Ileana, possibly Maria), but nothing came of it. Albert and Elisabeth liked Astrid very much after she and Leopold got to know one another.
Astrid and her siblings were seen as very desirable possible spouses for other marriageable royals following WWI. While Margaretha seemed pretty devoted to Axel of Denmark from the start, both Martha and Astrid were linked to several Norwegian, Danish, and British princes. Queen Wilhelmina of the Netherlands really liked their brother Carl as a husband for her daughter Juliana, enough so that some souvenirs were manufactured and sold in anticipation of an engagement (in reality, he and Juliana didn't like one another). This is unusual for a group of prince/princesses who were merely grandchildren of a monarch, but understandable given the dynamics of Europe at the time.