Hi pepo,
Yes, Mixture questions almost certainly will count towards your score, but the 'scale' of how much they will help/harm your overall score is LIMITED by the fact that you'll see so few of them. By comparison, your performance on other, larger categories should take precedence - since those larger categories impact your score so much more. For example, DS will have a HUGE impact on your score (since there are so many DS questions that you'll see). Easy-to-answer questions, as a group, will also count "a lot" towards your score (which is why every Test Taker needs to eliminate silly/little mistakes from their performance), but an individual Mixture question won't kill your score if you get it wrong.
GMAT assassins aren't born, they're made,
Rich