I try not to make assumptions about these things, though my methods lately have been biting me. I'll write within a notes document and gradually delete those notes as they are incorporated into the article. I originally started this to make sure every bit I gathered made it into guides, but I have no reference point when things come up as changed, buggy, or flat incorrect.
The 'garnish' ability may have got confused here. It raises the quality of baked goods, and works across the skills. But I noted that this does not raise the price of baked goods for retail purposes. I've edited that out. Checked other guides, and see them saying it raises quality too. I don't reference these because I don't like to copy possible mistakes (which is also called plagiarism), so something weird happened
Just made a discovery for its purpose though. I can definitely confirm that if you're selling baked stuff, decorating raises the value a whopping 50% and we already know that excellent has a huge boost in value. I can't get it to raise quality. I missed that fact, as in order to see a value for these you must go into buy mode and hold the item. You can then sell even higher on markup. Excellent fudge bars went from $93 to $139 - quite a leap! It makes sense that a pretty, decorated cookie will sell for more than a plain one.