Is participating in the international competitive programming contests and achieving an excellence degree graduation compensation for the low rank of your university when you apply to MPhil in Advanced Computer Science at the University of Cambridge?
The admission requirements and international equivalents make no mention of university rank. (Disclaimer: I haven't checked every single country in the international equivalents). You can check for yourself whether your degree results meet the required standard.
I haven't found specific statistics for CS, but the overall ratio of applications to admissions for graduate study overall at Cambridge is about 6 to 1. (The ratio of applications to conditional offers is about 3 to 1, but it sounds as though you have your results already and would get an unconditional offer or a rejection). If you meet the basic requirements you could expect an interview.
The interview is where things like participating in Informatics Olympiads may be useful. If you've mentioned it in the application, you may get a question about it. But that kind of thing is more useful to undergraduate applicants than to graduates, unless you can show direct relevance (e.g. good results in undergrad-level security competitions, when you've mentioned in your application that you want to focus your studies on security topics). The reason that it's less useful for graduate applicants is that you will probably have done some kind of individual project in the final year of your undergraduate degree, and that project tells the interviewer more about your interests and abilities than Olympiad results.