While Catherine proves immune to Thorpe's charms, we must remember she was given time for reflection, and he had a pre-engaged heart to attack. I blame his mother, for if she had made her appearance in chapter three, Isabella would have monopolised Catherine and informed her judgement of Henry Tilney and men in general in a way that would probably injure Henry's first impressions irrevocably, and improve John's chances considerably.

Boy-mad teenage girls (like the Thorpes)do tend to get together and egg each other on to the extent that a girl with no great reason to feel confidence in her attractive appearance or her discernment of its effects on the opposite sex, feels grateful to be allowed the opportunity to ride aimlessly around the town in a second-hand pick-up (or on the back of a 250cc motorcycle with one footrest and no helmet). If the owner of this vehicle should happen to be either a brothers friend or a friends brother, his offensive assumptions, life-endangering lack of consideration, preposterous assertions, allegorical nick-name and huge asses head are all so many endearing young charms when the message comes down the grapevine that "he is over head and ears in love with you".
If Catherine had found such a suitor among her brothers for Maria or Anne, I doubt his being foolish and obnoxious would outweigh the distinction that having a boyfriend gives to a girl who has never had a boyfriend before. In the social environment that these girls are in, a brainless chump with small prospects, like James Moreland, is better worth having than no boyfriend at all, and a misogynistic rake like Capitan Tilney is the biggest prize in the pond. Even in this modern day, one can see pretty girls attaching themselves to gorillas by similar devices. High spirits and hormonal turbulence, inexperience,lack of foresight/insight/introspection/ awareness/choice are all at work to accomplish it.
To be fair, it works the other way too - one might even say that Catherine Moreland has punched above her weight.