Dear Aspirant,

I am going to answer your questions in chronological order. First yes you can pursue CSE after your class 12th. Secondly, no your marks in class 12th won't affect your graduation.  Also, yes you need to get good marks in cet exam or whatever engineering entrance exam you are planning to give. Because grabbing a CSE branch in a good college is touch as it's very popular amongst students. After you get admission in CSE branch then you can learn whatever you want. You learn app development there is no big deal.

About mathematics, in my opinion by more maths you mean you do you need to be good in maths. The answer is yes. Mathematics is very important in your field. Do try and master it.

I hope this helps.

Good Luck