In pitching to an investor, in general, I'd say that you are better off with an idea with more customer validation done and a rough prototype than you'd be with a polished/pretty prototype that has no/minimal customer validation.
In pitching to potential customers, at a minimum, the prototype has to do enough to engage the customer, help them see (actually or help them easily imagine) how it solves their problem.
The thing to watch out for is muted reactions from customers, e.g. "Yes, I guess we could probably use this." Instead, you need to be able to elicit this, "Oh my God, this is great...by when can you make this a fully functioning version."
Without much context about the product/company (and whether its B2B or B2C, etc.), it is hard to provide specific advice/suggestions, but hopefully the above help give you an insight into my thinking. Happy to discuss more if you'd like to.