I found a large chunk of Johto's fillers, especially the ones in the middle, to be horrible, and in some cases, nearly unwatchable. Not only was the main cast flanderized to the point where they had monotone interactions with everyone, but every Johto filler followed the same boring repetitive formula over and over again, particularly the ones in, "Johto League Champions."
This is also where the humor of the series disappeared sans the Team Rocket fillers or some of the Pokemon-themed episodes. The writers couldn't make the show like Kanto was due to Takeshi Shudo departing the series and him leaving really made the difference know. The Pokemon on the cast at the time, particularly Ash's, were not handled well and almost never even appeared out of their Pokeballs. I remember we went long periods of time without ever seeing Bayleef/Cyndaquil/Totodile ever doing anything.
How exactly they were flanderized?
Because it didn't seemed to be case with Ash, Misty and Brock still had excellent dynamic between themselves not seeming forced or shallow. Chemistry between them contained sharp and witty humor, lot of banter and tension as well understanding between themselves standing up for each other with friendship with time becoming stronger.
I recall Ash was still naive and brash making full of himself in front of others like dojo master from Hitmontop episode, Duplica or when being interviwed by DJ Mary for radio show.
Misty was highly vibrant character there showing many traits such as love for water types, falling down for cute pokemon like Teddiursa, Snubull, Chinchau , blowing up at TR , Ash or other people stupidity(such as that small elder with Shuckle), when they battled for Totodile, who will win Whirl Cup or when they got late because of him . She showed plenty of snark teasing Ash about his battle abilities, big appetite in front of others or tricking him in wearing pokemon costumes(like one with Octillery). Plenty of adventurous spirit liking to try out new things(such as attempt to cook),trying to resolve mystery behind stealing food problem caused by Houndum, who hides under Gligar mask etc being very lively. Or caring side establishing great chemistry with Sakura finding things in common, Egan , kid who tried to catch Dunsparce etc.
Brock didn't seemed flanderized there either not exaggerating with flirting over girls like he did in DP showing lot of caring side when taking care of Golbat before evolving or his Pineco. Being nervous when meeting up with old friends like Suzie bringing back Vulpix, or shy asking Ash for help to get himself out of trouble he fall into(like kimono girls). Intelligent and wise preventing Ash or Misty from dropping in trouble etc.
And im not sure about what lack of humor your referring too? Because main cast provided lot of humor. We can see that in Ash brash and impulsive side being rude toward gym leaders like Whitney and Pryce not wanting to wait for challenge. In embarrassing himself in front of others using most experienced pokemon like Charizard against beginner trainers like Casey, not knowing what things which are common knowledge are like "Shihan" mistaking it for restaurant, not thinking before saying things. Such as adding even more salt to wound to trainers which lost like Marcellus saying how "he must be really bad trainer to lose so soon" etc.
With Brock trying to seduce girls being flirty toward them, in his fights with Ash over pokemon like Donphan and who is gonna catch it etc. Or his nervousness trying to run away from marrying Temaku, in his eccentric behavior licking other pokemon like Sudowudo etc.
Or in Misty eruptions over trainers which made fun of her lik Andreas or Dorian, when she find out she was Gyarados in horoscope, in fights with Ash whenever they got lost in forest or desert landscape. Being freaked out when getting surrounded by bugs. Trying to investigate things by herself and partake in something later regretting it such as beauty treatment in ninja village, embarrassing herself in front of others like breeder with Ninetales with all her questions telling her to get lost. Coming up with biting sarcastic inputs when commenting on Brock obsession over girls, Tr failure recognizing them behind their disguised clothes etc.
Johto had good share of its flaws, but characterization was definitely one of its strongest points with main cast having appeal and playful dynamic , truly feeling like real group of friends which had fun traveling together when actually center of attention was focused on them. If anything their friendship was actually strengthened there developing very strong bond between themselves, and giving out impression of companions which passed through many things knowing themselves for long time with interactions containing lot of comedy, sarcasm, tension and moral support even there
Johto was never meant to be treated like Kanto, but sequel to first two seasons with original trio being one of this region highlights.
Granted there are some Johto fillers I did love and find hilarious, but that doesn't make up for all the bad ones. But then again this is a saga where even the, "important" episodes are kinda meh, so there's not much to say in general. I stopped liking Johto shortly after Charizard/Squirtle left and didn't enjoy it again till some time in Master Quest if I'm being honest.
Fair enough, but quality is rather subjective. People have different criteria in what they will consider funny and entertaining, and what not. So even if you, me or someone third don't or do like something doesn't mean how someone else will share same view forming different opinion about it.