The only possible explanation for this movie's existence...
Please read what all I have said here. Don't respond with "tl;dr, but I agree", because then I'll know you're just being a jerk.
Maybe the studio had planned an Ace Ventura 3 for years, with Jim Carrey set to reprise his role. Same with sequels to The Mask and Dumb & Dumber set in development hell throughout the '90s. The thing is, Carrey became a diva and decided he was too good to do sequels to the movies that made him famous and thus he prematurely pulled out, and since he had that kind of star power, they couldn't stop him. So now New Line and Warner Bros. were left with movies ready to go into production, now that Carrey left, as he was probably one of the major reasons why they were in development hell. The production contracts had already been signed, so these movies had to be made one way or another. Hence, the producers were left with two options: 1) re-cast Jim Carrey or 2) rework the film so it can feature another character in the same universe. Son of the Mask chose the latter, and then came re-casting. I'm not sure how Jamie Kennedy got to be in the movie, I guess the studio had him under contract or they figured he was going to be "the next Jim Carrey". They tried to give him Carrey's schtick, but alas, it failed miserably.
The problem is that the original movies had been tailor-made for Jim Carrey, and when he wasn't going to be in the sequels, that put them in a tight spot. Sure, they COULD completely rewrite the scripts, but then the audience might not like it if the movie was really different from what came before it, so they tried to keep them close to the originals and figured, "hey, if we get some person who vaguely resembles Jim Carrey and give him that shtick, the audience will be satisfied." In addition to that, apparently the studios wanted to aim for younger audiences, so they tried to keep Son of the Mask and Ace Jr. "kid friendly." And when a studio tries to keep a movie family-friendly, that means they become scared to death of saying ANYTHING that might possibly offend a parent. Yeah, the kids don't care one way or another, but the parents are easily squeamish. Your average family movie audience now adays consists of either the Soccer Mom or the Divorced Dad, or sometimes Soccer Dad and Divorced Mom. The divorced parent is in need of a movie to take their kids to on the weekends during their visitation time. Since the other spouse is some control freak who doesn't want "that worthless bum" to have "their" kid for more than 3 hours a week, they choose the first kid-friendly thing they see. Who cares, right? Make the kids suffer for your own mistakes. The soccer parent is someone who wants the world to bring up their children, so they get them involved with more activities than one can stand, and they take them to a movie and expect a movie to teach them the finer things. When a movie fails at that task, the parent takes it out on the studio, because he/she is expecting that movie studio to raise their kid. The parents could do it themselves, but there are just so many more important things to do instead... like reading which celebrity is a crack-whore or who Brittney Spears is banging this week, watching fake reality shows with horrible performers, and my personal favorite, texting. 24/7 365, thumbing keys of a stupid little pocket-phone to talk to someone standing two feet away from you. You look like an idiot, by the way. And that, my friends, is why the movie industry of today SUCKS.
Bottom line: garbage like Ace Ventura Jr., Son of the Mask, and Dumb and Dumberer were only made because Jim Carrey the Diva backed out of the sequels, leaving the studios with his giant shoes to fill and three movies that had been signed into production. If you recall, there was a lot of talk about The Mask II and a sequel to Dumb and Dumber back in the '90s. Nintendo Power readers DEFINITELY remember the Mask sequel...or those poor souls who won a walk-on role in a contest, which they then forgot about when Son of the Mask came about.