Yes, I believe rap could be considered a form of poetry. This is only my opinion, but if you look at rap lyrics when they are written down, you may even mistake them for poetry at first. There are many forms of poetry that take shape in ways that look very different from the traditional iambic pentameter of a simple poem. Poetry is meant to expressive and open to interpretation.
Any rap lyrics that meet this standard, could be considered poetry. The fact that rap is typically set to instrumental music makes it a slightly different art form but take away the instruments and you have poetry. This is evident if you witness rap battles or other freestyle rap forms.