I tried adding examples to pumpkin hat, pumpkin head and pumpkin on head to try and visualise the differences out. All the example posts including if you check how posts are tagged using these look like they overlap. I found a few stand-out examples that could likely clarify the definition of the differences between these tags.
Pumpkin hat because it's an average hat with a pumpkin shape/theme.
Pumpkin head because it's a pumpkin being worn as or is the character's head.
Part of the problem is the poor definition given by these wikis and their tagging on posts. Pumpkin hat's wiki suggests that it covers all three because it says that it applies when it's a pumpkin, pumpkin-resembling hat, part of a pumpkin, a pumpkin for a head, a mask and a simple pumpkin resting on someone's head.
You have post #6877735 being tagged as pumpkin head because the head resembles a pumpkin (the hair is short-cut and orange to resemble the vegetable bit and a quasi-ahoge to show the stump) and probably because the point is they're jack-o'-lanterns given bodies. Most of these tags are all just about jack-o'-lanterns rather than pure pumpkins.
Pumpkin on head may sound like it also means pumpkin head but the way we differentiate "x on head" from "x head" in tagging is that "x on head" is for when something is on top of a person's head (their cranium) while "x head" means the person's head is replaced with that object. See animal on head and animal head.
Another thing to consider is that we already have food on head and fruit on head with a bunch of unimplied tags like apple on head, so it feels like these stray tags are giving too much granularity to a generic concept that could otherwise explode to have massive numbers of implications like holding tags currently do. They should all be collected and implied or destroyed in favour of food on head + the food tag in question, which would reduce how many things are at play.
I would consider post #8883375 a pumpkin head without being a jack-o'-lantern because it does not have the characteristics of the latter. Jack-o'-lantern specifically requires a carving in the pumpkin for the expression. Unless you get a weird post where a character looks like the one in this example post but has a face carved in the side as well, there could be some difference and split to jack-o'-lantern head but it lacks a wiki as well.
The definition overlap and overbroadness of pumpkin hat is one of the main problems here.
