Nadal is trying to transition his brand at the moment. He is getting older, his hair is falling out, his knees mean that he may not be in the game for much longer. If he just sticks with the 'young' image then his endorsements will eventually dry up because someone younger and cooler will eventually come along.
Federer is very lucky in this respect because his 'brand' - timeless class - is not tied to his age or his continued tennis success. In fact, his brand even has the potential to grow even more after he retires by capitalising on nostalgia.
If Djokovic wants to be commercially successful, he needs to fashion himself an image - something that he will be automatically equated with whenever someone sees him. As people have mentioned, I think the whole shirt-ripping thing is a bit of an attempt to build that warrior image. But it's not really something that is obvious in every aspect of his public life and his game so I don't know how successful it will be.
Basically I think his biggest problem is that his career is half over, and until recently he's never tried to come across as anything other than a normal dude. An effective brand requires years of cultivation, which for sports stars means they have to start very early.
I remember reading an article about Michael Jordan and Pete Sampras a while ago. In the 90s they were both handsome, rich, and dominated their respective sports. Both were being touted as arguably the GOATs in their field. Together they were the face of Nike, the same way Federer and Tiger Woods were in the '00s. But Jordan was very image-focused, whereas Sampras was basically just a nice regular guy who was really good at his job.
The result? 'Air Jordan' became a brand, 'Pistol Pete' did not. A decade later, Jordan's name is still selling t-shirts, sneakers and pretty much any other item of basketball merchandise you care to mention. Sampras, on the other hand, has a smaller commercial profile than Boris Becker.
Some players have a knack for marketing themselves, others don't. Djokovic, for all his advantages, seems to fall into the latter category.
As a fan of Djokovic, a poster may see as a tard instead, I much prefer Djokovic as himself instead of packaging himself to appeal to the vast market, like Mr. Federer and to certain extend Mr. Nadal. Not surprising as I am also a fan of Pistol Pete