How is the poem "Phenomenal Woman" by Maya Angelou a source of inspiration to women?

First, while this is a poem that some women may find inspiring, others may not. The poem received a substantial amount of publicity from Oprah Winfrey, a popular television personality, whose work tends to emphasize "inspiration" in the sense of an uplifting positive outlook on life, slickly marketed.
The aspect of the poem that some women find inspirational is the narrator's assertion that although she is not conventionally pretty, men still are attracted to her due to a combination of personal magnetism and a sort of visceral appeal and physical voluptuousness. As many women in the western world feel under pressure to conform to media standards equating self-worth and beauty with thinness and the traditional image of the fashion model, some women may find it inspiring that the narrator of the poem proclaims her confidence even to the point of suggesting that she evokes jealousy from "pretty women," as in the lines:

Pretty women wonder where my secret lies.
I'm not cute or built to suit a fashion model's size ...
It's in the reach of my arms,
The span of my hips ...

Although some women find this pushback against unrealistic expectations of body shape and size inspiring, there is a somewhat less inspiring side in that the poem's narrator does not challenge the notion that a women's worth lies in her sexual attractiveness and that a woman's self-valuation should be based on a sort of competition with other women to be stared at more by men as a sexual object. In that sense, this poem seems to fit well with the ethos of Oprah Winfrey's magazine O that serves as a platform advertising and selling beauty products, marketed to audiences of all skin colors and body types.
Personally, I would find a poem that located the value of women in their intellect, skills, and character far more inspiring than one that suggests that what makes a woman "phenomenal" is men staring at her as they would a piece of pornography.