I don't know that there's a "hard no" as long as you use it at or below the IFRA guideline for the type of product you're making AND you live in a country where the rules allow this or don't regulate the amount of flavoring.
I have no idea where in the world you live, but to give you an example, I understand the rules are more stringent in the UK for the amount of fragrance in soap -- 3% might be the max no matter what IFRA advises. I'd expect the UK to be just as picky about flavorings in lip balm.
Another issue about using higher amounts of flavoring in a product is that this ingredient is a liquid. I'd expect at higher percentages of flavoring, the product will be softer. You should be prepared to tweak the formulation if needed to get the right consistency. And for the extreme example you gave of using a flavoring at 20% of the total amount, it's possible that no amount of tweaking will give you the right consistency.