I feel like there needs to be a standard judging system that posts are based off of. I mean we kind of take each mon in a different direction, basing our opinions off of a plethora of factors, from the official art to the in-game sprites to fan art and that skews how people nominate certain cutiemons or open up a window for bashing from others. For instance if we judge Nincada by its in-game sprites, people might say it has a certain air of cuteness; by its official art, there's probably few followers, but by some of its absolutely adorable fanart, evangelism to the Nincada cult comes full circle. Then we're stuck between "it's ugly here but cute here" and that's annoying.
so what are we actually talking about? official art? sprites? models/renders? fan art? Fan art can really make ANYTHING look cute, even Pawniard which is normally a lifeless chess piece with nothing that has the potential to express any emotion other than its eyes.
Very valid concerns. Which is why, for the most part, I don't really believe in the "show photographic evidence" part, or, would take it with a grain of salt. I place more stock in arguments, especially ones that have solid basis in canon, like on Bulbapedia or some other source. You can often easily argue things about a Pokemon's fluffiness and personality without having to dredge up fanart. I've been doing this for things like the much-maligned Happiny and Flabebe.
At the same time I acknowledge that pictures have a certain sway. I think that fanart to be used for argument's sake should be as close as possible to how Sugimori or an official artist would draw it. For instance, while I absolutely adore Natu, I had to filter out fanart that made Natu overly fluffy or gave him silly beady eyes which aren't how Natu is. I love Joltik to death and believe it is well deserving of its S-rank, but 95% of the fanart tries to oversell its chibiness/moeness/fluffiness and I would never use it to make a case for Joltik.
This is an example of a good fanart that showcases Joltik's cuteability:
http://trakker.deviantart.com/art/I-see-you-hiding-X3-184210523
This is not:
http://fox-feathers.deviantart.com/art/Joltik-Nom-197029118
The first fanart fudged Joltik's color a little, but otherwise is pretty faithful to how the little fluffy arachnid looks like. The second one is an overly-chibified ball of fluff that is cute enough but is a bit far off from being Joltik--overly circular eyes and undersized nommers.
(This is not to say that people shouldn't draw Pokemon in a different style, no, not at all. While creativity is nice and all it's a bit difficult to take an argument for a Pokemon's cuteness seriously when it's based on fanart that oversells.)
In all, I think this is the pecking order for cuteability arguments.
1) Official art. This includes the anime, TCG, manga, in-game sprites, etc.
2) Arguments based on canonical sources.
3) Fanart.