Whether or not it's funny is just subjective. It often gives me a chuckle. And besides, it's part of who Ash is as a character that he often gets in those situations. He's a character that goes into new endeavours with a lot of enthusiasm and excitement, he's impatient and he doesn't really think things through. He's the kind of person that just dives in blindly, messes up, and learns from his mistakes to grow. Why is it such a problem that Ash is most often encountering these situations? Why should there be some kind of hidden tally that keeps track of how often each character is the butt monkey which should line up equal in the end? I mean, compare him to a character like Lana, who is very different. Lana is more the silent observing character. She observes and thinks things through and then, through her own innate talents and skills perseveres. Due to the nature of her character she just naturally doesn't enter those same situations that Ash does. Lillie is similar to Lana in a way, but with different fears. Lillie is mostly made fun of with her fear of touching pokémon and her poor abilities in sports and other physical activities, but she won't enter in the same butt monkey situations Ash does because she is simply not as reckless. Kiawe is made fun of plenty of times, usually for his tendency to take everything very seriously and to get carried away competitively (like at the sleep over when he had Turtonator destroy the living room). Sophocles is also made fun of occasionally, he's fallible in his anxiety and poor physical abilities, and Togedemaru doesn't seem as attached to him as the pokémon of the other characters are to their trainers (although that's also Togedemaru's personality).
Let me put it this way. If all characters were to be the butt monkey equally and made fun of in similar situations, what sets the characters apart?
Also out of all of them, Ash gets WAY more vindication. Sure he's a butt monkey in many episodes, but almost all of them have him redeem himself and do something intelligent or badass.
Granted sometimes maybe this is why the girls feel far less accomplished. They don't get the lows, but they don't get the highs either. Out of the girl characters Lillie seems like the only one XY-onwards that's allowed to struggle or lose at anything (and thus be made to improve), even in situations the character should logically be inexperienced (e.g. non-battlers like Serena and Mallow always getting through battles without a SINGLE hit, while pros like Ash will sell against even TR if it makes the story work better). Butt monkey or failure roles aren't just ratio thing so one doesn't look more pathetic than the other, it's something that makes them sympathetic. A character HAS to lose or suffer humiliation at some point otherwise they look too plot armoured, or alliteratively, start to become a bland invincible ace like Ilima, not to mention never being challenged or defeated means they never have to improvise or get better. They remain on top through the same bare bones skills, and thus end up boring.
Add to that, sometimes I feel like the writers are knowingly doing this and the ratio thing actually starts to take over. As in it looks like they bawk from making the girls lose or look pathetic in too many things, but at the same time they cap their abilities and involvement because they don't want them to end up looking far more competent than Ash as a result of struggling less (as mentioned, giving them completely clean winning streaks while downplaying their battle count and still making it blatant they're amateurs). I'd rather the girls had the same winner/loser tendencies than Ash than just be harmlessly mediocre and inactive.