Just to make this clear from the start: I am not a TLK3 "Hater". In fact, I like it a bit more than Simba's Pride (that movie had such potential to be really, really good, but that's another post) But, here's the reason I think this movie gets a lot of whack.
A very critical problem with a lot of Disney's direct-to-video sequels is that they give off the impression that they have no understanding and/or respect for their source material. This is most distressing for the older Disney movies seeing as the company still owns the rights to the film so can do whatever they want with the characters, though the original people to work on said film are either dead or retired so have no say in the matter.
An example of this lack-of-faith to the source material is the showing of human's faces extensively in "Lady and the Tramp 2", seeing as in the original film all the humans are generally seen from the knees down (i.e. from a dog's perspective) This gives off the impression that the makers behind the sequel either didn't study the original in order to determine the best way to make the transition from movie one to the sequel as seamless as possible, or they just didn't care, and as a massive fan of Disney, animation and movies as a whole, I find that pretty unacceptable.
Now, to apply that to TLK3, let me first say that I don't think the use of humor is what people have a problem with. The main problem in TLK3 is the undermining of the dramatic and serious tone that TLK seemed to pioneer in terms of Disney: having Timon and Pumbaa be "present" throughout the entire first act of the movie was a pretty ridiculous choice in my opinion. Fair enough to show Timon's back-story, fair enough to show how Timon and Pumbaa came to find the jungle, but anything else was not needed. It seems that almost every deep and meaningful or dark and dramatic moment of the original TLK was spoiled somewhat by forcing Timon and Pumbaa into the scene i.e. Circle of Life, Mufasa talks to Simba (deleted scene), stampede, CYFTLT etc, and I think it was a poorly informed choice on the part of the film makers.
Having said all that, though, I still find the final film of the trilogy very enjoyable to watch. I remember the excitement the first time I saw it when I was about 8 or so (I still remember the excitement of watching the trailer for the first time (on the TLK Special edition DVD!
) Also, I feel like the majority of the hardcore hate for it comes from within the fandom - I don't think it troubled the casual Disney-consumer that much on release.