The entire world of the Super Mario games seems to be alive. One plausible theory for why is that the Mushroom Kingdom is full of homies, like in the manga One Piece. If Mario ate the Soul-Soul Fruit, he may have created the Koopas and orchestrated the kidnapping of Princess Peach.
In One Piece, one of the Four Emperors, Big Mom, ate a Devil Fruit called Soul-Soul Fruit that gave her the power to transform inanimate objects and animals into anthropomorphized creatures called homies. Using this power, Big Mom transformed much of Whole Cake Island's nature into homies. The result is a landscape that has a very similar feel to Mario worlds, with smiling clouds and hills.
Perhaps Mario was bored with the monotonous job of a plumber and orchestrated all his adventures using the power of the Soul-Soul Fruit. This would explain the similar appearances between the worlds.
It also offers a more satisfying explanation than the official story. In the Mario series, none of the creatures in the Mushroom Kingdom seem to support Princess Peach. Almost every living creature seems to support Bowser's plans to imprison her, aside from the Toads.
The 1985 Super Mario Bros. manual says players don't see supporters of the Mushroom Kingdom because they have been transformed into “stones, bricks and even field horse-hair plants” by the Koopas. This paints an uncomfortable picture. When Mario breaks a brick in the game, he is killing citizens of the Mushroom Kingdom that he may have indirectly imprisoned by creating the Koopas after eating the Soul-Soul Fruit. By examining just the first Super Mario Bros. game, this theory takes shape.
More obvious creatures that fit the Soul-Soul Fruit theory are the Mushroom retainers and the Little Goombas. Both are mushrooms that have been brought to life and the latter are described as mushrooms who betrayed the Kingdom. Similarly, it is easy to make a case for Bullet Bills, as they are inanimate objects that have become sentient.
At first glance, the Koopa Troopas just appear to be turtles, which would not be enough of a transformation to suggest the Soul-Soul fruit was needed to create them. However, Koopa Troopas, Koop Paratroopas, the Hammer Brothers, Lakitu and Bowser are all members of the Turtle Empire, so there appears to be enough anthropomorphism in the group to distinguish them from regular turtles.
Most of the other creatures seem to be neutral in the war, neither supporting the Mushroom Kingdom nor the Turtle Empire. They don't appear to talk, so they are likely bystanders caught between factions and not homies created by Mario. As far as Piranha plants go, that might just be what happens when the plumbers in the Mushroom Kingdom neglect their duties.
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However, not all of the Mario creatures could be controlled using the Soul-Soul fruit. Soul-Soul Fruit does not work on humans, so Princess Peach must be real. Perhaps Mario created the Koopas and instructed them to capture the Princess to win her heart by saving her from the evil Bowser. Maybe he orchestrates the whole plot of saving the princess to woo her.
The best evidence for this is how easy the Bowser fights are. Bowser, the evil ruler of the Turtle Empire, conveniently stands on a rope bridge in battles with platforms above his head to allow Mario to jump over him. To make things even easier, Bowser also places the axe that can destroy the bridge conveniently by the supports. He seems to spit his fire out in a random order, not truly trying to hit Mario. It all seems suspiciously easy for Mario to win.
The best evidence against this theory is that Mario can die at all. Homies should not be willing to kill their leader. However, in One Piece, homies have been known to question the judgement of Big Mom and can be somewhat independent. Perhaps seeing Mario kill their fellow comrades enough times encouraged them to more seriously take up arms against him.
