It is in generally poor taste for an employee to bad mouth a product they offer and/or sell. Everything offered onboard an aircraft is there because of a contract or agreement, as such, employees who denigrate such a product are operating counter to their employer's wishes. It's also poor practice to criticize a customer's choice, scold, or otherwise guilt trip a customer about their choices.
It is worth a complaint, despite other comments in the thread about it not being a big deal. It's not the soda itself, it's the issue of employee behavior when the employee is the face of the company.