Judging what is "worse" is tricky. Of course, if the alternative is proposing policies that don't fit with your ideology, you are going to judge them badly ("how can they be good politicians, they haven't even understood what is obvious to me"). But it does not mean they are worse, it just means that they are not representative of your ideology. In a healthy democracy, the community should accept that there will be people with different ideology, and that there will therefore be political parties representing them.
I think this is the problem: politicians representing the ideology X can afford to be mediocre because they have some room before "voting for mediocre politicians that still agree with what I believe is the best" is worse than "voting for non-mediocre politicians that disagree with what I believe is the best". If a party ends up with mediocre politician, it is its own failure, and the other parties (even if they themselves have bad politicians) are not responsible: the other parties represent other people. If the party that represents your ideology is managing to put forward mediocre politicians, then, it is your failure for letting that happens or accepting this situation.
(I'm not saying that you are incompetent: you may have failed because it was impossible. Still, it's certainly not the responsibility of people who have a different ideology)
In short, in a healthy democracy, there will always be a "worse" party. It cannot take away the responsibility of the party you have voted for for being bad. It's a bit like having giraffes and lions at the zoo, and having the lions saying "the meat is full of maggots and really bad quality, but the alternative is having the vegetables that suits the giraffes, so, it's the giraffes' fault".