I've seen several central flower shots that were negatively critiqued for being centered. I've seen several central flower shots that received no mention at all for being centered.
Conclusion: If there's no fault to be found with exposure, focus, DOF, etc then the old "too-centralized-in-the-frame' commentary can be mentioned .
It's too often too difficult to apply a rule of composition to an image of a single flower.
Don't anguish over a compositional non-issue and devote your efforts to Sharpness, saturation, shadow detail, blowouts in the highlights, light dynamics and direction, and proper focus.
IMHO, the rule-of thirds is the very least issue to be concerned about with your image and most flower shots in general.
Do you think your shot is soft? Do you think flowers are ok to be on the soft side in general (some photographers do--I do not). What is the issue with your shot that you think would be the least objectionable? What do you think is the most objectionable aspect of your image?
Personally, I think your image is as good as the most that are submitted for critique, not quite as good as the best, and Way better than some.
Cheers