Yes, of course senior grades matter. However, what matters more is how a person has done overall, especially throughout high school. A lot of people apply and are accepted to college before they complete their senior years, and some of them are accepted before they even start that year. That makes it hard to say that the grades matter that much.
Colleges do like to see what courses you're taking in your senior year, though, just to see whether you are taking it way too easy or whether you remain dedicated to your education. It won't stop you from getting in if you're getting lazy, but it does matter. On the other hand, you might not have done that well through your first three years of school but then you really did well in your senior year. That might not be enough to get you into the college of your choice, but it will certainly make a difference, and it's something that you can talk about in your college interview.
How you present yourself may matter more to some colleges than a few bad grades, but don't count on that to get you through.
