Based on his dates with a werewolf and an antelope, both of whom he actually treated with respect, it's possible Johnny may just be acting sexist because he thinks that's how to impress most women (and is too dense to notice how rarely it works), and is a great guy apart from that. (According to this interpretation, he wasn't sexist toward the werewolf or antelope because the usual "rules" of romance didn't seem to apply.)
There's also the interpretation that most of the women he hits on are just as shallow and don't want to bother getting to know the real guy under the bravado. Though it must be said that no one is obligated to get to know someone who walks up and sniffs them. Actually putting your hands on someone you don't know also tends not to be very endearing.
Johnny's father is never seen or mentioned in any episode, and the only adult male in his life (Pops) is of questionable moral character. Did a lack of a father figure or other positive male role model growing up keep Johnny from developing the skills needed to become a proper man?
When his character is viewed as a whole, Johnny isn't a bad person. He's vain and shallow, yes, but he also has admirable personality qualities when it doesn't involve trying to pick up women. But a shallow person generally is terrible at first impressions. Plus as the creators have said, Johnny does get girls, but they purposely don't show his successes because that would ruin the joke.
There's also the fact that, at least after Flanderization kicked in, Johnny genuinely doesn't know how to actually romance a woman, presumably because he's never gotten that far. In one instance, a woman on a cruise who was genuinely interested in him (or rather, the ring he was wearing) offered to take him to the captain's cabin for a little fun. Once they got there, Johnny was a lot more interested in playing around with the ship's wheel and control panel than the beautiful woman who was quite literally throwing herself at him. So as mentioned above, he may genuinely believe that cheesy pick-up lines and acting like a selfish stud are simply how men are supposed to find girlfriends, and even then, he doesn't even know what romantic partners do with one another.
In the episode "Charm School Johnny" where Pops makes a bet with Carl that he can turn Johnny into a gentleman in seven days in which the loser would have to perform a Mexican hat dance for twenty-four hours straight, Pops ends up losing the bet after Johnny mistaken the Royal Duchess for Carl and tosses her into the punch bowl and then Carl shows up afterwards apologizing for being late. Given Carl's not surprised reaction when he sees that Johnny failed at becoming a gentleman, it's possible that Carl showed up late on purpose knowing that Johnny is stupid enough to mistaken the Royal Duchess for him so that he could win the bet against Pops (which is backed up by the fact that Carl just so happened to bring a sombrero for Pops to fufill his part of the bet), or he just simply showed up late by accident and just wasn't surprised as he knew that Johnny would not become a gentleman and he was also happy that he won the bet against Pops.
(and is too dense to notice how rarely it works), and is a great guy apart from that. (According to this interpretation, he wasn't sexist toward the werewolf or antelope because the usual "rules" of romance didn't seem to apply.)