I know the difference between a joke and a serious one but what are the actual features that is implemented in the hack that is considered a joke and that is serious?
Another question must a serious hack be a serious hack all the way or can there be some jokes in it?
You can’t exactly pin-point the element of joke and “serious” hacks, but usualy, joke ones use a wide arrange of portraits with varying styles, some even featuring characters from other games, like having Mario from Hotel Mario as a playable character.
In this way the game indirectly tells the player not to take the game too seriously.
But some may opt not to, and just stick with splices or custom mugs (for example, the Heroes We deserve, and many more).
Joke hacks often have memes and a silly plot, in order to flesh out the comedy present in the interaction between characters, but again, this doesn’t mean all joke hacks are like this.
Some choose to follow a more structured plot akin to a normal hack, some start simple to devolve in a real adventure, and so on.
To answer your 2nd question, it’s not uncommon for serious stories to have some comic relief (or comic relief characters) in order to let the player/viewer/reader take a breather from all the plot happening around them.
A simple example might just be filler episodes in relatively serious series;
just don’t go overboard, and know when to make a joke and when to remain serious.
A “joke” hack will, as mentioned above, often feature a mixture of multiple characters from various, seemingly unrelated sources all mingled and mashed up in one single story, whose structure/dialogue are often fairly simplicistic and contain an element of the “absurd” in their structure, which often tends to serve as sufficient evidence to the reader that the story is not meant to be taken all that seriously (though the stories themselves absolutely still can take themselves seriously in spite of all the surrounding absurdity). They may also feature similarly “absurd” gameplay elements to enhance the experience; this often taken the form of either several units with wonky/unusual stat spreads, weapons with drastically changed properties/new weapons with similarly absurd stats, or perhaps most prominently, various hidden items and other content across possibly several maps even. These hacks are also often on the shorter end so as to not overstay their welcome, as no matter the amount of delivered comedy, it can eventually run the risk of growing stale, especially since their “stories” are often barebones structure wise and their comedic effect lasts in fleeting moments.
A serious hack on the other hand, has similar/consistent artstyles and often tries to tell an actual, cohesive story with an often serious tone (as is typical of a fire emblem story). As mentioned above, they can sometimes contain comic relief, but it must be done moderately so as to slightly break the immersion/give a bit of a feeling of suspense; constant gloom and doom would annoy/bore even the most patient of readers eventually after all. That said, these hacks can also still feature other funky elements like units with wonky stat spreads/growths and hidden items as mentioned in the above point, but they’re often done in a more sensical manner without neccesarily relying on hidden meta jokes and what not. Their chapter titles are often actually serious/indicative of what might happen during said chapter, unlike joke hacks that may take more liberties with their map titles.
Hope this helps!