There's "obvious" bad ideas like trying to sell sand in the desert.

There's the bad (underwhelming) ideas, maybe that's the most common of them, it's probably the 'trying to make a weird idea sellable'. Like 'french fries are amazing but unhealthy so let's try to sell potatoes cut as fries, but baked, with no salt'

There's also quirky ideas as bad ideas, which is the way most people go and what PG probably meant. Not to mention sometimes your idea looks like the same as a failed one already but the devil is in the details.