There are a number of public health reports coming out recently from the UK and the US citing increases in mental health problems , particularly in young girls as a result of social media and the messages it sends.
The focus on the need for validation from peers, the idea that being beautiful, thin, rich with lots of followers is how to "win" at life is very damaging. For the first time in a while, there is a rise in the number of suicides in teenage girls, and a sharp rise in self-esteem and body issues.
Instagram itself has only been around for about 7 years. Social media is like a giant social experiment with no parameters. No one knows what the long term effects will be on individuals but it doesn't look positive.
However it is not just Instagram. Musical.ly is as porny and inaapropriate as all hell, Snapchat an excellent way to bully your peers and share inappropriate images, Yubo/Yellow is tinder for teenagers, Sarahah essentially an app to anonymously harass your peers .... none of it is great.
But your petition idea will achieve nothing. Instagram perhaps should up the age recommendation for sure, but again as it's not enforceable (other than accounts getting deleted because the user is underage if reported via their web page ) there's really no point. There is no way for Instagram to check the account holder details , and most children lie about their age to get particualr social media accounts anyway. Why waste your time?
For younger teens and children parenting properly is the answer. It's not the fault of the app , the schools, the government or anything else - it's the choices people make as parents. Educating yourself, adequate supervision, not succumbing to the "but everyone else has it" mentality , would be a start.
Keep in mind that the age restrictions these apps provide are only recommendations, not legally enforceable and are the BARE minumum they can get away with. The app warns that there is nudity, sexual content etc before downloading. It was not designed with tweens /teenagers in mind.
#freethenipple turned into a penis horror show in days, and any trending or popular topic searched has roughly a week at most before it's inundated with porn. The moderators cannot keep up with the content.
As said above, there are settings that can be used to blur content, and block bad words which might be an idea for your niece.
What would be better is her parents had not allowed her to get the account in the first place, apparent appropriate age or not. If half the parents I deal with bothered to educate themselves about the various social media apps they let their kids loose on, social media wouldn't be half the problem it is.