Is there a word for a low-grade actor/writer/singer/etc who's gained a widespread attention and support?

Solution 1:

Because s/he is (over) hyped / hyped-up.

The subject didn't reach its height on its own merits. Rather, it was boosted by a hype machine, marketing scheme, fad or viral campaign. Some external factor to the person in question.

From the Cambridge dictionary:

Overhype: Make exaggerated claims about (a product, idea, or event); publicize or promote excessively:

"the film was overhyped by the press"

Solution 2:

There are numerous terms, such as fad

An intense and widely shared enthusiasm for something, especially one that is short-lived and without basis in the object’s qualities; a craze: prairie restoration is the latest gardening fad in the Midwest

Oxford Dictionaries Online

Similarly

• craze
• flash in the pan
• the rage
• passing fancy
• the latest thing
• whim
• novelty
• gimmick
• hot shit (taboo and very negative)