Saying that a utility monster means a "creature that is somehow more capable of experiencing pleasure (or positive utility) than all others combined" is vague, because it doesn't mean a creature that's just more capable, it's a creature that's a specific kind of "more capable". Just because human beings can experience more utility from the same actions than nematodes can doesn't make humans into utility monsters, because that's the wrong kind of "more capable". According to your own link, a utility monster is not susceptible to diminishing marginal returns, which doesn't seem to describe humans and certainly isn't a distinction between humans and nematodes.