is Grendel's mother a representative Germanic women?
This is an interesting question. The answer to this question is "no." We can arrive at this conclusion in two ways.
First, if we look at the role of women in the poem, Beowulf, then you will be able to see that they are worlds apart from Grendel's mother. The most notable woman is Wealthow. She is not a monster in the poem. She is the legendary queen of the Danes, who is characterized in the poem as a hostess, a force of civility, and a symbol of unity among the warriors.
Second, if we look at Grendel's mother, her characteristics do not cohere well with Wealthow's. In many ways, Grendel's mother is the opposite. She is even less human than Grendel. This should be expected, because as the poem states, she and Grendel are from the cursed line of Cain. Moreover, of Grendel, the poem states, "[he is] malignant by nature" and "never shown remorse." This can be applied to Grendel's mother as well.
The only commonality that Grendel's mother has with Germanic women is that she cares for her offspring. I suppose, even monsters love their children.