Why do Manga and Anime defuse so often tragic situations? Moments that can be tragic/serious are often immediately defused by some hilarious moment and some stupid face of the characters. I personally find that disturbing that a plot cannot focus on something serious for a little while. Is that a stylistic choice or just a way to target a particular audience (i.e. children).
From your comments, I gather you don't like the use of Anti-Climax in M&A as a medium. Though it can be jarring, Anti Climax can be great if used correctly.
Anticlimaxes can work well if it's clear that the subversion of audience expectations is the point, either for humorous purposes (such as the Anticlimax Cut) or as a more serious commentary on the genre of the work. If the Anticlimax is unintentional, however, or if the author's purpose just isn't sufficiently clear to the audience, the result is serious audience frustration. Use caution.
I think the root cause of this problem is discussed quite well on TV Tropes.
Not all anticlimaxes are intentional, though. Oftentimes, they are caused when the story writes itself into a corner. Other times, it's caused when the writer realizes that their planned solution just wouldn't make sense compared with the logical one. Sometimes, it's caused when there are teams of writers that don't communicate very well. The planned resolution of a Story Arc is nullified by another writer, who might have written out the plot device intended. Sometimes, in the case of film and television, it's caused by budget constraints or unexpected cancellation. It's rather rare for unintentional anticlimaxes to show up in single works, usually popping up in long serials where there isn't a chance to unobtrusively go back and rewrite some pivotal moments to set up the proper climax.
The problems stated above are quite common in the Manga and Anime industry and thus as you seem to have found out can lead to disappointing endings to a great story.
I would believe this is something of psychology where it makes the situation funnier; it's like after tasting some salt, the sugar will taste much sweeter. If I try to explain this using a more understandable way, think of the number line: the funny side is to the right and the serious side is to the left and you are at the 0 point right in the middle feeling no emotions. When things start to get serious, you move to the left and feel serious at for example -3, but then they drop a joke which makes you laugh and you move to the right. Because 0 is the neutral point where you feel no emotions, you have to move past 0 to feel happy so you have to be at least 1 or above. The funnier the joke, the higher the number. Now, because you are starting from -3 rather than 0, it gives you 3 more than if they just throw the joke at you in the start. This can work the other way around as well. This is mostly why they use this technique.
If they do it wrong, or when they see people like you who don't enjoy these sorts of things, it doesn't work. From what I have seen, these are mostly used in slice of life and shounen manga/anime. In shounen, they will get serious, then someone makes a joke, then they get deep and serious again. If you look at genres such as horror, this "trope" practically doesn't exist. An example would be EVA (I believe): there is no funny bits in there, it's serious throughout the show, but this might differ from person to person.