Our experience has been that a classically bad poem marries a trivial or dubious topic to an effective if superficially solemn technique. Generally speaking though, the most common example of bad poetry tend to revolve around a lofty subject hobbled by inept execution. James McIntyre (Canada's bizarre "cheese poet") would exemplify the former notion, while "all up" bad poets like William McGonagall and Julia Moore would be examples of the latter category.
Truly bad poets, the Edgar Guests, the William McGonagalls, the Amanda McKittrick Ros's have a curious consistency throughout their writing careers their first works are as reliably laughable as their last.
A classically bad poet in our experience, never grows or changes, their poesy remains a battered limping thing all the way.
So when it comes time to categorize this type of material it's important to note the difference between a bad poem that comes out of poor subject matter chosen by an otherwise able lyricist and material generated by a lifelong hack.
Both produce "bad" material by our light but descend to those depths by different means.