In The Book Thief, Liesel Meminger is described as a young girl on the verge of adolescence. Born in 1929, she is around 10 years old at the start of the war in 1939.
Her physical appearance is relatively important, as Nazi Germany was a society that rewarded certain types of physical characteristics and punished others.
Her hair is blonde—"a close enough brand of German blonde."
In the context of her story, it seems that the author is showing that she doesn't quite fit in, even though her hair is an acceptable color.
Another description of her appearance in the book is of her smile. It is described as a "smile that was starving." Considering her situation, and the constant risk her foster family endured while hiding Max, she didn't have much of a reason to smile. But every once in a while, whether it was in conversation with Max in the basement or in other moments, she was able to let her emotions show.