Not at all. Your ask the recruiting team (HR) to postpone an interview, but the interviewing team (consulting) makes the decision. They won't know you even postponed, nor will they care.
Remember - consultancies want the best people. If you haven't been able to practice correctly, for one reason or another, you won't do well at the interview so (1) everyone will waste their time, and (2) consultancy won't see the "good" you and make a decision based on bad data. They'd much rather you postponed and camed prepared.
tl;dr: no downside to asking to postpone; worst case, they tell you they can't accomodate and you interview at the date initially set.
