Regarding asking for writing about some courses in my CV in which I got bad grades due to my non interest in those courses
My general advice in such situations is to stress the positive in any application. Show the reviewer that you are a good candidate for success both in background and in work habits. Don't focus on explaining things that aren't positive but be prepared to respond when/if asked about them.
While the competition for a slot in a good university is very strong, nobody is perfect. And you won't know how competitive you are until you apply.
If you have a fairly narrow focus you intend to follow in graduate studies, focus on that and how you are especially well prepared to work in that field. Be enthusiastic, not apologetic.
Caveat. I don't know how your grades are interpreted by your own university, not how they will be seen when you apply. But focus on the good stuff, not the bad.
I'm not a mathematician, so take this answer with a grain of salt, but ...
I strongly recommend not framing your grades as "I was not interested in these courses so I did badly in them" (assuming that 6/10 is indeed not good - I have no idea how the read this grading scale correctly).
As a hiring committee member, my mind goes immediately into envisioning you as a student who is only able or willing to put the work in on things that directly interest you. This is generally not a desirable property in a graduate student, as no student ever gets to do only things they love. Even in grad school, and assuming that your project actually is of high interest to you, there will likely be aspects that aren't. Maybe it's the course you are teaching, maybe it's part of the coursework you have to do, maybe it's something completely different - not all of these are crucial to your success, but most advisors would rather not see you fail in any of these aspects. Further, most of us have seen too many students (temporarily) fall out of love with their research subject (remember that grad school can be quite a marathon), and during these times it is crucial for students to be able to soldier on.
You can of course write that your lack of interest in these subjects was holding you back, but I encourage you to also give an indication that you understand that this isn't a good excuse. What is done is done, but you need to convince the committee that you are not going to do badly in every task in grad school that's outside of your core interest.