Maybe the best way to introduce a character like Mr. Wormwood, who's so vivid and slimy, is to let the narrator do it himself:
In came Mr. Wormwood in a loud check suit and a yellow tie. The appalling broad orange-and-green check of the jacket and trousers almost blinded the onlooker. He looked like a low-grade bookmaker dressed up for his daughter's wedding, and he was clearly very pleased with himself this evening. (5.5)
Mr. Wormwood's outfit is blinding. Blindingly bad. It's almost cartoonlike in its color scheme ("a yellow tie" and a suit that's "orange-and-green check[s]"). The narrator's comparison of him to "a low-grade bookmaker dressed up for his daughter's wedding" says it all: Mr. Wormwood is tacky and totally self-absorbed. Despite the fact that others see a pretty sleazy guy when they look at him, Mr. Wormwood thinks he looks downright sharp.
A bookmaker, in this passage, refers to a bookie, or a gambling man. (It's kind of a funny word choice considering how much Mr. Wormwood likes to read—that is, hardly at all.) Though he may sell cars rather than take bets, Mr. Wormwood still has the same sleazy mindset. Let's take a deeper look into why you shouldn't trust this guy as far as you could throw him. Just like you don't want the Trunchbull for a headmistress, you definitely don't want Mr. Wormwood as your car salesman.
The bottom line is that Mr. Wormwood is "[a] cheat and a liar" (5.36). As a secondhand car salesman, he only sells used cars, which gives him the opportunity to mess with each one, and lie about his products. For example, his special selling tactic is usually to say that a car's previous owner, a little old lady, hardly ever drove it. He puts sawdust in the car engines, and uses a drill to make the speedometers run backwards. He'll knock fifty cents off a price to make the car sound cheaper. Yeah. Real classy, dude.
To top it all off, he's a hypocrite. He criticizes his own daughter for being "[a] cheat and a liar" when all she does is complete a math problem in her head correctly. Maybe he assumes she's cheating because he's a cheater himself. Maybe he can't accept the fact that doing things properly—not taking major shortcuts or trying to get ahead by breaking the rules—actually works for some people.
He is all the things he accuses Matilda of being. The more he lies and cheats, the meaner he is to his own daughter for her supposed cheating. Meanwhile, he claims he's a great salesman and thinks he's really smart and has discovered the best sales method of all time. He sees his marketing spiel as clever, rather than, oh, you know, illegal. The more he treats himself as a genius, the more we realize he's the opposite of that. The man's a no good, dirty crook.
However, others can see right through his lies. Miss Honey tells Matilda that he's a "criminal." The cars he's selling aren't just doctored; they're not legit purchases to begin with. He's a lying, cheating criminal—and to top it all off, he's a bad parent.
To this list of inspiring qualities, we can also add general egomaniac and übermeanie to Mr. Wormwood's dossier. One of the nicest things he says to Matilda is that she's "'an ignorant little twit'" (2.6). In other words, he's giving the Trunchbull a run for her money in the mean, meaner, meanest contest. Matilda is an amazing individual, but Mr. Wormwood can't be bothered to give her the time of day. Instead, he likes her brother better, just because Mike's a boy. How's that for fair reasoning?
Mr. Wormwood also thinks he's the most important person in the family, so he likes to make a grand entrance every now and then. And by every now and then, we mean all the time:
He was incapable of entering any room quietly […] He always had to make his appearance felt immediately by creating a lot of noise and clatter. One could almost hear him saying, "It's me! Here I come, the great man himself, the master of the house, the wage-earner, the one who makes it possible for all the rest of you to live so well! Notice me and pay your respects!" (6.15)
Mr. Wormwood uses up so much energy on himself that he doesn't have any left for anything or anyone else—especially for Matilda. He thinks he's the big cheese, but honestly, he's nothing but a bad apple.
It's no wonder he cares so little when asked whether Matilda can stay behind with Miss Honey, instead of going to Spain with the family. He's more concerned about missing the flight than with whether she comes or not, and when the family drives away, he doesn't even glance over his shoulder to say goodbye. To which we say, good riddance, ya jerk.