I was also going to say no, but everyone's saying it haha.
- No, reports denied due to "Already punished by Sentinel/Staff" isn't a bad thing. It's just because they cannot punish the same player twice.
- If you're a helper applicant, no, doesn't affect at your chances.
- If the report is denied because you lack the evidence, then it's possible that it could affect at your helper application.
(I don't know if you're a helper applicant, I just put that example haha)
Conclusion: No, isn't a bad thing. You're helping to keep cubecraft safe after all.