Giving up before even starting: life without a PhD
It's quite likely you'll be able to get a job in industry with a mediocre Masters.

Think about it this way: there's a sizable population that never finish their Bachelor's degree. Among those that did complete their Bachelor's, a sizable fraction never do a Masters. In other words, you are already quite rare. So you'll probably be able to get a job in industry, if only because you don't have many competitors. Another way to think about it is, given that you're doing Masters, I'll guess that you did quite well during your Bachelor's. Odds are you know people who did worse than you (i.e. had "mediocre grades"). Did they manage to get a job? You can check, although I predict the answer is "yes". They might not have gotten their dream job or one that pays as much as they want, but they got something.

It sounds like you are asking this question because you're unfamiliar with the job market. Therefore I suggest searching for jobs in your vicinity that require a Masters degree in whatever field you're studying. It's imperative to know, because then you'll have some idea of what you're going to be working as after you graduate. If you find there're lots of vacancies, your chances of getting a job goes up.

One more thing: don't view not getting a PhD as "giving up". I know quite a few PhD holders who, in retrospect, think they should not have gotten a PhD.


  What's the point of producing mediocre (research) results?


I don't know, do you? If you don't know either, why are you even considering it? You're in charge of your own happiness; you have to alter your own fate.