Here at CSM we have a yellow "Pause" button, which means "know your kids." This book, like most of Roald Dahl's, should be labeled "know yourself." Some adults hate it for the same reason that kids love it -- it shows a good, smart child overcoming evil, dumb adults. It has ridiculous, cartoon violence, not meant to be taken seriously, where no one actually gets hurt. It has a black and white view of the world: the good are all good, and the wicked get their comeuppance at the hands of giddy, delighted children. If any of this bothers you, if you think children's books should always have a respectful attitude to adults in general and parents in particular, keep it out of your house, because griping about it will make you look just as nasty and clueless as Matilda's parents.
If, on the other hand, you can enjoy this type of humor, it's a harmlessly guilty snicker you can share with your kids. It's a silly romp, a good read-aloud, and a mild challenge for middle graders to read themselves. Either way, it will have them giggling and feeling immensely satisfied at the ending (which bothers some adults even more than the rest of the book). So make your choice and then live with it, because railing against this book is not going to do you any good at all.