There are two reasons why Greta’s leadership of the youth climate movement is not a distraction, but a positive mobilising force. One stems from her message, and the other from the nature of global protest movements.
Firstly, in Greta’s message, she explicitly refrains from endorsing any political ideology or policy initiative. Her entire message is basically, “don’t listen to me, I’m a kid, read what the scientists are saying.”
During her congressional testimony in the US, she deferred to the IPCC special report on global warming. “I do not want you to listen to me,” she said, “I want you to listen to the science.”
This deference to science makes Greta’s role as a leader of the climate crisis as unobtrusive as possible. Rather than a guiding force, she is a spotlight, drawing attention to the best available united science.
It is true that critics and commentators will focus on mundane details like her appearance and mannerisms to detract from her message, but this is not a result of her being a distractive presence in the movement. It is a result of a concerted effort of climate change deniers to keep the attention on anything but the available science.
The second reason why Greta’s emergence as the face of the climate crisis is not a distraction but a driving force for the movement comes from the nature of social movements. The Hong Kong protests have been successful in building a movement without leaders. However, this was not to keep the focus maintained on their demands and to avoid distraction. It was to keep the authorities from rounding up the leaders and imprisoning them, thus leaving the movement fractured and disjointed.
In fact, the movement’s lack of a figurehead or public-facing figure is now holding it back. The movement has no narrative or arcing story. The protests themselves are not a story. For a movement to be successful and sustain attention, it needs someone to verbalize the struggle and to hold the international media’s attention. Without a leader to tell a story, the movement becomes nothing more than a wishlist of demands; media attention fades and, ultimately, change remains beyond the movement’s reach. 
Greta is able to give the youth movement a narrative. Her calls of “this is all wrong,” and “I should be in school,” give the protests context and a narrative. This narrative draws more people to the movement and holds the world’s attention. It prevents the context being drowned out by the noise of the protests.
The news cycle is fleeting. Protests can only sustain attention if there are developments. Without Greta as a face, the school strikes would have faded from the front pages as soon as they started. With Greta, the movement has a face that can sustain global attention.
Her exploits like sailing across the Atlantic in a yacht, speaking at Davos and staring down Donald Trump all provide opportunities to put the cause back on the front pages and renew the conversation around climate change. In this sense, far from a distraction, Greta is part of the reason why the movement enjoys sustained coverage.