The point of a protest is not to rationally convince the rulers to change. By the time a protest occurs, the “rational” and “legitimate” options have been exhausted. Protests are a show of power by subjects to their rulers, and a threat that more serious consequences will follow if nothing changes. The fact that a mass of people have assembled and are taking action at all is sufficient for a protest to be effective IMO.

So I don’t think “basic” slogans make protests less effective so long as they don’t oversimplify the demands of the protesters. Similarly, creative slogans can definitely help protests be more effective if they sharpen the message they intend to deliver.