Being liable ("subject to liability") has to do with A's ability to seek compensation from B for a wrong that B has done that harmed A. A customer cannot be liable for damage done to himself, though he may well be responsible. The company might be liable to the customer. In turn, the software developers might be liable to the company, or to the customer, if the developers were negligent. If the customers were somewhat responsible, that (contributory negligence) could reduce the liability of the company or the developers, though for what you describe it's hard to see how there could realistically be contributory negligence on the part of numerous customers. Depends on what you mean by "is hacked" – usually, if an idiot uses "password" as his password, we don't say the company was hacked. Customers don't have information that they can negligently expose that leads to hacking of a company database.
And of course, lest we forget, the miscreants who steal the data are also legally liable.