I think there's more than just the poet's chops, though -- the subject has to be the right size for a sonnet: too complex, and it won't fit in that basket. Too brief is also a problem, but it's not all that difficult to spin out material with imagery and metaphors and so on without it actually coming across as padding. (In this, it's not as strict a container as, say, a sestina, which is dueced hard to pad out, as well as being harder to stuff beyond capacity.)

Yes, and yes, about technical proficiency not being enough. "Not doing anything wrong is not enough -- what matters is what you do do right."

---L.