Is viewing nakedness considered a sin even if it does not cause arousal, and how can one atone for this sin and ensure the acceptance of repentance in this blessed month?

Looking at 'awrah (private parts) is forbidden, whether it incites lust or not, because it is a potential cause for inciting lust. One must immediately cease looking at what displeases Allah, resolve not to return to it, and regret past sins. If this is done, then repentance is complete. Sincere and truthful repentance is accepted and erases sin, for "The one who repents from sin is like one who has no sin."