Idiot ball: A moment where a character's stupidity fuels an episode, or a small plot line.
There are several types of idiot balls. I'm going to talk about a few of them and show how they relate to the Ranma manga. First PIS (Plot Induced Stupidity), PIS is when for reasons of a plot a character acts stupid. Quite often a character will fail to properly use their abilities, intelligence, or powers when generally in a similar situation they would. A movie example would be a character who through out the movie had perfectly one shot killed every villain encounters a major villain and instead of shooting this villain in the head like he did every other one he misses or instead shoots the villain in non lethal places. PIS is seen more often in long running series and in comedies.
In the Ranma manga, Ranma suffers a major case of PIS during the Gambling King arc. In that arc Ranma is unable to play cards, fails miserably. He has no ability to bluff, he doesn't notice an obvious worn out card, he falls for and picks a card just because it pushed up some, he can't control his own facial expressions, and he can't read the facial feature of the person who gets quite obviously sad when a bad card and incredibly happy with a good card. Ranma's abilities and intellect falls quite a bit for the comedic plot. This lack of abilities is contradicted by the rest of the manga, that doesn't mean Ranma doesn't occasionally do stupid things (he's overly competitive and impatient which does result in him doing things that in hindsight he'd view as being dumb).
Inability to bluff -Ranma has shown the ability to lie through out the manga. He convincingly lied about destroying instant boy water, lied every time he pretended to be someone he wasn't, lied to Shampoo about his gender, etc., I can go on for awhile.
Unobservant - Ranma not picking up that the guy was cheating and not noticing the card is contradicted by the rest of the manga. While occasionally Ranma will not notice his surroundings and jump on someone his incapability to recognize the card, that the guy was cheating, and not noticing the guy's facial expressions doesn't make sense. This is a person who after just viewing the Umisenken once was able to copy it, viewing the tea ceremony old lady in the proper scooting position was enough to copy it, saw through other peoples disguises (Principal Kuno's, Gosunkugi's, Happosai's), figured out Akane was attracted to Dr Tofu, etc.
Ranma is generally not slow on the uptake. In fact he is generally the very first person to figure things out. He figured out that Kuno was the principals son before anyone else did, figured out the source of Ashura/Rouge's power, figured out that the cursed spatula was really an iron, etc.
Ranma is very competitive and absolutely hates losing. This is shown through out the manga. Yet Ranma is shown to play cards several times both before that story arc and after. If Ranma had been consistently losing, he would have been doing everything in his power to improve or would have stopped playing. We see him play cards with his classmates in school during multiple arcs, for example at the beginning of the Romeo and Juliet arc and we have seen Ranma not losing against other people at cards. Here is an example: He beat both Ryoga and Akane while at Ryoga's house pretending to be Ryoga's sister.
Had Ranma lost or had trouble because he was unlucky that would have been very believable. Ranma's luck through out the manga was very bad. People would walk in on him in compromising positions, dogs would attack him for no reason, he'd encounter magical items, inexplicably slippery banana peels show up in strange places, etc. Had the Gambling King been good at cards, especially with his cheating (which allowed him to beat everyone including Nabiki), or had Ranma not been shown to be able to hide his facial expressions, lie, be observant, etc. I would not have had a problem with that part of the story arc (there are other parts of it that I seriously dislike).
Now onto CIS (character induced stupidity). CIS is similar to PIS, except the character is doing/acting/being stupid not for reasons of plot but natural limitations that characters impose upon themselves that reduce their ability to use their own skills and powers effectively. In other words the character is acting dumb because that's how the character is. The limitation can be because lack of intelligence, morals inhibiting the character from using some tactics, etc.
Akane's inability to swim and how she is very stupid about it is in my opinion CIS. Akane's inability to swim and her thoughts about it is not a one time thing. While it is part of the plot, it is more CIS than PIS. There is not one instance of this there are many. In one storyline she thought that you have to breathe in water to swim. She is unable to even use floating devices. She drowned in water that was ankle deep. Ranma's impatience is the main cause for his CIS moments.
In addition to PIS, CIS, and the idiot ball the later manga also had character decay with some characters becoming more two dimensional. Genma, for example, in the early/mid manga would not have gone to the lengths he did to get something worth only $20, an hour or two of work (Genma did have a job remember). Stealing and attacking Ranma and his wife was in character but going to all the effort he did for something worth so little was out of character at least for early manga Genma. Anime Genma is another thing entirely who doing so would have at least made sense since he was willing to sell Ranma for a small amount of food.