Things might change for you in your gap year, and some sixth will still recommend alternative courses and education if you fail mocks
Also if you get good grades in mocks, still apply to unis in year 13, you can have an offer and then defer it for a year so you can go on your gap year knowing that it is sorted without having to worry about applying to unis on your gap year. Plus your school will help you with your application and as soon as you leave they have no obligation to do so.
In short yes, try to do well and yes they do matter