It depends on your definition of natural. Baking powder is a combination of several things: an alkaline component (which is often sodium bicarbonate aka baking soda), a starch, and acid salts.
Baking soda naturally occurs on Earth in a mineral form called nahcolite, however, the majority of baking soda found on grocery-store shelves in produced artificially because it is cheaper and easier than harvesting nahcolite.
The starch that is primarily used is cornstarch. Cornstarch is, well, the starch of corn. It is naturally occurring and is not modified.
Now for acid salts. I will try to explain this in the simplest terms I can in case you aren't chemistry inclined. Acid salts are certain kinds of acids that have been partially neutralized (pH increased). Because the acid is only partially neutralized (it's still an acid), you end up with a product (what's left when the acid and neutralizing agent are combined) that almost always contains a metal ion, at least one hydrogen atom, and an an atom or molecule with a negative charge (an "anion").
Conclusion: I will assume your definition of "natural" is "naturally occurring on our planet." According to this definition, baking soda and cornstarch are both completely natural, however, acid salts are not. Baking soda is found naturally on Earth, cornstarch is simply a part of the corn grain that is harvested, but acid salts are created by a chemical reaction. Though this reaction could (and probably does) take place naturally on Earth, the acid salts used in baking powder are created with specific characteristics in mind and are not harvested naturally.
I apologize if this is a longer answer than you wanted but I wanted to be thorough so you could understand every aspect. Bottom line: No, baking powder is not natural due to the acid salts found in it.
organic
Quite inorganic as it is not really big on carbon hydrogen bonds.
Sulfur powder can be either organic or inorganic, depending on its source. In nature, sulfur is typically found in a mineral form and is considered inorganic. However, sulfur can also be produced synthetically through chemical processes, in which case it would be considered organic.
Baking soda, chemically known as sodium bicarbonate, is classified as an inorganic compound. It is a salt composed of sodium ions and bicarbonate ions, and does not contain carbon-hydrogen bonds typically found in organic compounds.
it is organic
Lecithin is an organic compound.
inorganic
it has an organic macrocycle and an inorganic metal at the centre
there are organic acids and inorganic acids
it is inorganic
Inorganic
it is inorganic