Is "ad populum" ever seen as an idiom for "popular", or can it only mean "to the people"? For example, if something is considered popular, would "ad populum" be an acceptable phrase to use?
Ad populum is such a common phrase that searching for such a particular usage could be quite tedious. Offhand, I can think of no example.
You could simply use popularis, though it's more usual to use a circumlocution to describe someone or something as 'popular'. A phrase such as in vulgus acceptus has the precise meaning; similarly studium populi habere and coram populo gratus (and, no doubt, several others) might serve well enough in many situations.