Describe Mileva Maritsch's frustrations with her marriage to Einstein. Is it Einstein with whom she is displeased, or something else?

Mileva is not displeased with her husband, but with her situation. It is reasonable to assume from her character that she feels stifled by marriage; the duties common to running a household prevent her from pursuing her scientific and academic interests. Perhaps she resents her husband a bit for allowing her to fall into this situation, but he is not the primary source of blame for her unhappiness.