Is it okay to ask the hiring manager for feedback on myself even if I have already got the offer? I just really want to know how I perform, especially compared to other candidates. Also, I believe the feedback might be helpful as it might identify my weakness, etc.
My recommendation is no, don't do this. Why? You're sending the message that you're already looking ahead to your next job interview. This will give them the impression that you don't plan to stick around very long, which is not what you want to do when you're starting a new job.
As a comment says, you were the best candidate for the job - at least who hasn't turned them down (yet). Consider this confirmation that you interviewed well.
If you accept the job, then once you've started - within conversations about how they want you do your job - it is probably okay to ask if during the interview they noted particular strengths they want you to utilize or weaknesses they want you to work on.