Not normal at all - if anything I’d have safeguarding concerns for your niece. I say this as a mum of a 17 year old DS who has thankfully avoided that kind of shit so far. I do find social media really fucking disturbing & it is a massive cultural shift (especially for those of us who grew up before mobile phones & the internet..).
However dick pics at 14, trolling paedophiles, sending graphic personal photos etc are all thankfully rare and sadly dependent on peer group & probably a lack of parental control/involvement and influence.
Mostly though the kids are a product of the environment they grow up in, which thankfully OP you still have influence over now and for a while longer. This will be the deciding factor in whether your little girl goes in the direction of your niece (not nieces fault at all, just circumstance presumably).
This is an article which explains what I’m trying to say far more coherently: www.ncbi.nlm.nih.gov/pmc/articles/PMC4777050/
It’s not a forgone conclusion - the culture your niece is describing is a reality for some now, but it is by no means the only way. There are nice kids and nice environments around.
Also all teen generations in the last century or so have faced their own challenges. The late 80s/early 90s were brilliant, but we were flashed at on a regular basis, rape wasn’t taken very seriously, mental health problems didn’t exist (I.e weren’t recognised, acknowledged or treated) so they often led to awful outcomes. Homophobia, racism and sexism were rife and commonplace to the extent that they were normalised and laughed off and people still used offensive terms for anyone with any mental or physical disability freely and with no comeback or judgement. You are just describing the evolution of this shit.
We didn’t get ‘dick pics’ we just saw weirdos wanking at us in real life. We didn’t ‘troll peados’ but our mothers did tell us that when men exposed themselves to us in the park we should ‘point, laugh, make a disparaging comment such as “I’ve seen bigger things crawl out of an apple” and then run (my sister & I have laughed about what on earth we would have done had we pointed, laughed and ran but forgotten the put down....maybe run back say it & then run again!?).
We didn’t send pictures of our genitals to other people (it would have cost a fortune in Boots and the pocket money wouldn’t have covered it..) but an early conversation about that will help your DD, to avoid it as sending them screams low self esteem/the need for external validation.
What I am saying OP is that the world can be and always has been a shitty place - it’s up to you to give your DD high values and expectations, to establish clear boundaries and communication & do your best to make sure the people who become ‘her people’ are decent and kind and strong. You’ve got plenty of time as your DD is only 5....over to you.