The first miniatures painting competition I entered into, I painted my subject with realism in mind and that got some respect, but didn't win me the prize. I looked at the first, second and third place winners (I got honorable mention) and saw that all of them incorporated a cartoon-like paint scheme. I proceeded to teach myself how to do this and before long my stuff started to resemble the cartoon schemes that seemed to be the "craze".
See the thing about mini painting contests is that it is more an "art" contest than a contest of skill. What one judge or group of judges consider "good" another may find to be utter crap or not worthy of a top 3 place in a contest. The judging is subjective. "Realism" or "cartoonish" really boils down to a judges own preference.
Also look at the level of competition. There are usually only 3 places in any given category. Well, what if there are 4 or more entries that "deserve" to place? Then it comes down to really nit-picking the pieces apart.
Changing your painting style to match the preference of a judge(s) can be a risky move as I've seen tastes change from year to year. One year it appears as if all you have to do is make a fig from Nurgle and you'll place, another year it is dreadnaughts.
Plus, take into consideration the contest you are at. Is it a GamesDay? Origins? GenCon? I have found that GenCon is the most open of the group when it comes to a mini painting contest. Any manufacturer and various styles have taken top honors in the past. Plus there are the manufacturer's awards as well.
At Games Days, well, all you get is GamesWorkshop overload. I have had some success (well, two first cuts, "somewhat" success!) when entering a GW contest by entering something from a fairly new line of minis, or something that is not done very often (avoid Space Marines, they are EVERYWHERE!).
...all that to say that I like realism, but it seems that the cartoonish paint jobs seem to catch more eyes.
I would have to disagree on this point. When talking about harlequins, they are SUPPOSED to be brightly colored (based on the background fluff). But I have seen "realistic" looking minis win awards. Heck I have won some awards (Adepticon, GW hobby centers) and I consider my style to be VERY dark and realistic.
Bottom line is: Everyone paints for different reasons. If you are looking to win awards, then maybe changing your style with the hopes of matching the likes of a judge, is the way to go. If you paint to sell, then there are certain techniques that collector's look for. If you just paint because you would rather spend more time gaming, then the paint jobs may look rushed. Just do what you love.