I don't think there's any subject I'd reject as inappropriate for any form (and, indeed, would be tempted to consider it a challenge if someone decreed somesuch off limits) -- it's more a matter of whether the writer in question has the chops to make the form serve the subject. I remember reading a collection of sonnets some years ago that I found really, really boring, and that experience nudging me toward recognizing that technical proficiency in itself is not enough: it can be either or both structure and sauce (i.e., make a topic more interesting, because of the tension added by the form, or provide a spring for the poem to reach a height it might not otherwise), but those things by itself are still not enough.

(She says, glancing wistfully at a basketful of first lines...)