What defines a 'bad' monster? For me, it generally has at least one of these three qualities. First, it shatters verisimilitude by being either 'jokey' or just really hard to visualise or imagine. Second, it has some nuclear-grade special ability that can only really be avoided or circumvented by a successful saving throw rather than player intelligence. Third, it is just boring, usually because it is too much like a lot of other monsters, or because it has no obvious role beyond being a benevolent quest-dispenser or GMPC.
These qualities we can call, for shorthand, silliness, unfairness, and boringness.
On this basis, I would say that the Worst Five MonstersTM in the 2nd edition Monstrous Manual are:
5. Banshee. I am not averse to the concept of this monster in principle, as the concept of a banshee as it exists in folklore is deeply eerie and evocative. And actually the Monstrous Manual entry is nicely written and provides some good ideas for use of a banshee in a campaign region. But the monster itself is high in boringness (its role overlaps too much with that of the ghost or spectre) and unfairness (it gets to just show up, scream, and then everybody might die).
4. Cloaker. The picture in the Monstrous Manual does this monster no favours, but it is intrinsically very high in silliness, both through shattering verisimilitude (try picturing a flying cloak with glowing red eyes attacking somebody in your mind's eye and tell me it doesn't immediately transform into a scene from a cartoon) and for having no obvious justification for its existence other than surprising adventurers. And then you have the fact that for some reason it can emit magical 'moans' of different intensities. Now try imagining that: a moaning, flying cloak with a face in the middle.
3. Faerie Dragon. I just think that the last thing that any D&D campaign needs is a creature which 'thrives on pranks, mischief and practical jokes'. Practical jokes are visual, for one thing, and are not funny when being verbally described, but the more important issue is that joke monsters are like campaign cul-de-sacs. A random encounter with something which simply intends to 'wreak mischief on passers-by' provides no adventure hooks, nor danger, but simply acts as a distraction or speed-bump. The faerie dragon is therefore both silly AND boring.
2. Ki-rin. There are too many of this kind of monster in the Monstrous Manual, and they all bleed into one: couatls, lammasu, shedu, sphinxes...all of the same: flying benevolent sky dwellers who descend to the the world below to smite evil and help out the PCs (presumably on the basis of their taking part in some pre-ordained quest or mission). I am fully on board with the idea that there should be powerful good entities in the world if there are to be powerful evil ones, and that enterprising players should on discovery of their existence seek them out for aid, but there is too much of a duplication of roles her and too much of a stink of 'plot' about the ki-rin in particular. The boringness is off the charts.
1. Sea Lion. Just stop it.