EVERY designer believes that he/she is creative. To design, in the Jane Austen sense of the word, is to have planned intentions. A designer always has this planned intention. To be able to get so far, you have to be creative.
To assess a person's creative element, you would need to assess the level of sophistication of the executed design. Aesthetics and presentation do add to the overall mark, but disagreements in this area are bountiful. So, I would say the basis of judging a good design, is the logic, application and level of development that displays an aesthetic suitable to it.
How then can you tell a designer that they lack the creative element, when there are no opportunities to conceive any ideas? Conceptualisation and creation call for creativity. The final design element will be based on the initial concept and from there you can judge the level of creativity.
The vantage point is essential in order to judge the popularity of a design. Based on the specifics and the ideologies of the people and situation that the design is applied to.
This is where design philosophy is important. Wacky, out of the ordinary designs may appeal to most people as cool and fun. Others may prefer simplicity, elegance and sophistication. Both designs may be equally good, but of a different philosophy.
So when you try to tell a designer that they lack the creative element, beware of the pitfalls of that sentence. Are they that bad at bringing an idea to life? Or is it the difference in design philosophy that fuels your judgement?