It's a pretty good first shot at story and was very enjoyable. Will be happy to see more of your world in the future

For a first fanfiction, I have to say this is not good as a oneshot. However, if it were an actual, full-fledged story, it would be a lot stronger and overall make more sense, as well as being a better story overall. The plot was decent, especially since it was smut. There were a few grammatical errors, but the main glaring issue is the fact that it is a oneshot. The level of foreshadowing and detail put into it aren't really fitting with the format that a oneshot should follow. The story's ending includes several details that seem to suggest there's more to the story. Thus, it doesn't exactly come to a resolution as it should. I can recommend two different solutions to better your story.

The first is to continue the story. In truth, the plot points you really need are already there. You could easily lead into a second chapter with Peach either getting kidnapped, or already having been kidnapped. She could find a way to bring Flora Piranha back and run away with her, attempting to change Mario's view of Flora after she's established herself as co-queen of the Piranha Plants (technically speaking, she would be Princess's Consort, but you could probably disregard that, to be honest). If you choose this method, then I would suggest taking what you have already published and adding more depth to it. It still has some of the traits of a oneshot, after all.

The second option I can recommend to you is to edit the story in order to make the plot flow better as a oneshot. To do this, I would suggest reworking the ending to give it more of an air of finality. More specifically, you should put more focus into the battle between Flora and Mario, possibly even including some of Flora's inner thoughts so as to better express how hard Flora is fighting for Peach. Also, change Peach's reaction to the situation. Make her cry, or have her resent Mario for murdering Flora. Maybe even have her slip into a depression that affects the Mushroom Kingdom. Those changes would effectively translate to a reader that there was no way to bring Flora back.

Of course, there are other options and actions you could take to improve this story. The continuation idea was one that immediately came to my mind when I had finished reading. That's not to say that this story can't function as a oneshot, however. You, as the writer, can take whatever path you see fit when it comes to improving this story. With all this talk of improvement, however, I don't want you to think that your fanfiction is bad. As I said earlier, it is solid. It just needs a little bit of brushing up and revision.

In conclusion, the choice is yours in how to improve this story you've written. Either choice I suggested could help you to improve upon your story, but don't think these are your only two options. Whatever choice you make, I can say that I, for one, will support it.

Sincerely,
Lava Blarg