You need a strong character because merchandising is all about association. Take something like a football boot, that has nothing to do with the footballer in question(his pictures not on it), but its still an association. It's the same with the N7 logo. Without the Shepard character, the logo would have no meaning.
From what I've seen so far the Inquisitor looks about as a generic as they come , which is hard to market.
I disagree. People love the N7 logo because it's a visually appealing logo that they associate with the entire game, not just Shepard. Being that most people play something other than the default Sheploo, they aren't necessarily picturing that one face every time they see the N7 logo. I think the reason that N7 wearable merch does so well is because Shepard can look like any human being, therefore any of us can look like Shepard. When you put on an N7 hoodie, you are Shepard.
A visually strong character is one way of marketing that can create a good market for merch, but it's not the only way. People don't buy merch with Pokemon badges on it because of the protagonist of the latest game. They buy it because those symbols represent the overall experience of the game for them.