I would not favor a static method in Qobj for plot_qubism (and other plot functions) because they are often quite specialized (like in this case, which only works on kets) and therefore cannot be considered a function that is relevant to a general Qobj. This is mostly a question of personal preference, but if plot_qubism where to be a static method in Qobj then nearly all functions in qutip should also be Qobj static functions.