Unless you have already had a couple of resits, I wouldn't even think about what the airlines might or might not think about failing a couple of ground school exams. Ultimately, the material is not that difficult to get your head round. There is a ton of it, but that can all be taken care of with a lot of hard work. Put in the study hours, make sure you're doing well on the practise tests, and I can assure you the results will come.
Why on earth would someone consider slacking off or doing 'just enough to pass'? This is an attitude I think is all too prevelent (in general, not to you personally two-speed). Going for a first-time airline job is tough enough as it is. You want as much going for you as possible, and the only way a 85% or even 90% average is going to hurt you is in missing a few nights down the pub. It's rough if you have a job or two on the side as well, but sacraficing a year or two of your life will pay dividends later.
Half the battle is getting an interview. Once you've got that, you know the airline ultimately wants to hire you, otherwise they're wasting their time. Getting good marks will only help you get an interview, so go for it.
-abracadabra