I think we've all encountered times when "doing what's realistic" isn't necessarily what's best for a story. Whether it's in terms of events, characters, music, or whatever, sometimes something "unrealistic" can have a stronger emotional effect on an audience than something that's realistic. It might be more inspiring, more horrifying, more funny, and so on. It has a lot to do with audience expectations - a surprising concept may provoke more of an emotional response than an expected or predictable one, even (or especially) if it's implausible.
Of course, an unrealistic thing in fiction is just as easy to make as a realistic thing. When broken down, a fictional thing is a series of stats and elements given a general title, not an actual logical subsystem. For example, there are plenty of jokes that rely on the idea of something that's weak in real life being strong or powerful, such as:
- The Killer Bunny from "Monty Python and the Holy Grail".
- The Elephants, and later Carp, from "Dwarf Fortress".
- The Hoe of Destruction from "Ultima 7".
- A good number of weapons from Final Fantasy, including brushes, cards and dice, dictionaries, and handbags.
- Chainmail bikinis that are considered as protective as regular armor.
What is the underlying theme here? There is an overt mismatch between the creature or object and its capabilities. However, these are entirely unrelated things: in a game, an object's visual appearance and name have nothing to do with its stats. The difference between fighting a dragon and fighting a rabbit in an RPG can be broken down to (a) what it's called, (b) what it looks like, and (c) its stats. Those three things are not connected, but because we have expectations about them, we expect things to match up and be "genre-consistent".
The more abstract a game or system is, the easier it is to have mismatched concepts. In Final Fantasy, weapons basically just have an attack power and special effects. It's just as easy to give a harp or a flute a high attack power as it is to give a sword or axe. Things like reach, weight, and so on never come into play - and even if they did, it would be just as easy to inaccurately represent them. This was represented most clearly in the ZODIAC RPG system, where (at least in one edition, I'm not sure if this is true anymore) weapon improvement was not limited by type or class - instead, each character had their own system of weapon advancement and they could say their weapon was whatever they wanted.
This creates a strange situation. On the one hand, it's funny to be mauled by a rabbit, or beat someone to death with an umbrella, because it's going against expectations. It's unusual, and hence not something you'd expect to be able to do. On the other hand, it's really just a name (and maybe a graphic). Therefore, an appeal to realism is being made - but only for the sake of subverting it. Without the basis of reality, there's no reason for a super-powerful gardening tool being funny or unusual. It is only by appealing to prior experience and expectations that the joke can manifest in any way.
Similarly, something being outlandish or fantastic is also relying on our known experiences - and a side effect of this is that if the audience doesn't know what to expect, what's "fantastic" may not even be recognized. Here's an example: in the game "Ico", the queen's castle is totally unrealistic. I don't just mean in the "it makes no sense" way, but in a specific way where it's presented as being the work of magic, rather than just an unrealistic design choice on the part of the artists. It was not until this was pointed out in an analysis of the game that it clicked for me. Hence, there are two views of something being unrealistic:
1) An unrealistic thing is an error made by the authors, writers, designers, or whoever. They don't know what to expect or what something would really be like, and they made a mistake because they felt it would be "cooler" to do it that way.
2) An unrealistic thing is still grounded in the logic of the setting - it's exceptional within the confines of the setting, as well. It's explained by magic or advanced technology or something, but the characters are as surprised by it as the player is, because it goes beyond the limits of what they understand.
The former is distracting, because it undermines the logic of the universe in the same way that any other error does. The latter is reinforcing, because it brings the audience's perception of events in line with the characters'. However, the latter also requires consistency. Being able to tear a car in half is only impressive if the average person is, realistically, not able to do that. It is the contrast that makes it impressive, not the act itself. Similarly, being assaulted by a carp is only "funny" if a carp is not normally considered able to do that. If "carp" in a given setting are giant fanged water-monsters, then there's no contrast to make.
Really, this is a lot of the same logic that I've explored before: for something to have meaning, at least in-universe, it has to be contrasted against a baseline standard. Things can be "better" or "worse", but there must be an average to set expectations. If that baseline average is set according to real-life things, the audience can relate more easily by drawing in their own pre-existing expectations and understandings.
To Sum Up:
1) Regardless of whether it's done for reasons of humor or drama, "unrealistic" things must be compared to "real" things to be considered different or unusual.
2) Without the expectations of reality, "being unrealistic" has no purpose or effect.
3) A distinction must be established to distinguish intentional unrealism (for justified dramatic effect) to unintentional unrealism (i.e. "they made a mistake).