I honestly don't think you're a monster, not in the slightest. I also don't think what you did would have been scarring. I might be a little more laissez faire about nudity than some people (and I know there is a difference in how it is viewed here in Europe/Britain and in America) but I also think that it is an important distinction that your intention was not sexual and that there was no contact involved in what happened whatsoever.
I literally feel you have *nothing* to feel guilty about, no more than someone who streaks at university or exposes their breasts at a music festival should feel guilty i.e. the behaviour is a little inappropriate and maybe coarse, but at worst it will make people a little uncomfortable or feel irritated, but it's highly unlikely to be traumatic IMO. The fact that your niece is happy and outgoing seems like a good sign to me!
I think it's interesting what you say about Lena Dunham and Josh Duggar. Firstly, what you did doesn't even approach the same universe as what they did. In both cases what they did was clearly possessed of a sexual element and was on-going over several years. It also involved a significant element of power expressed over their sister/s. Lena Dunham has said that she basically views her sister as an extension of herself, which strikes me as a major red flag. Also, I think what disturbed people about what Dunham admitted to was the jokey, off-handed, almost gloating way she discussed what she did - it was very clear there was no remorse or even understanding that what she did was inappropriate or wrong. Josh Duggar's apology was better IMO but it was very much framed in terms of him stopping because he was worried about being caught. Plus it seems like his family did a lot to cover up what was happening and did nothing to support his victims or remove him from the household or anything else.
That said, even in the rather serious cases of Dunham and Duggar, I think think people have gone way too far in calling from their life-time imprisonment, their execution, or even just in the confident insistence that neither of them are to be trusted around children and are 100% pedophiles.
What they did was certainly child-on-child abuse and this can be just as devastating for the victim as an adult abuser. But I still think it is important to remember that children are still developing and can change. This doesn't mean they can't be held accountable, but I think this accountability can and should take a different form to the accountability we would expect a full-grown adult to suffer. In Dunham's case I think she needed urgent therapy that explicitly addressed what she was doing. I also think she needed to be separated from his sister. In Duggar's case I think a more legal response might have been appropriate... but I still don't like the insistence that a 14-year-old should be on the sex offenders register for *life* or put in prison for many years. That doesn't help anyone and it neglects the youth and inexperience of the offender.
I also think that the sheer degree of hatred and venom directed towards both figures (while somewhat understandable) is partly political. Dunham is a poster girl for the Left. And Dugger is a post-boy for the Right. As such, people on the opposite side of the political spectrum have generally been the ones who have been the angriest. Also, hypocrisy is galling and both are terrible hypocrites. Dunham primarily frames herself as a victim (which may also be true - perpetrators can also be victims... every human has the capacity to be both at different times of their lives or even simultaneously) and is a spokesperson for a sexual abuse and trafficking charity. She also talks a lot about the evils and horrors of sexual abuse without ever acknowledging that she herself is a former abuser. Dugger has spouted inexcusable homophobic and transphobic rhetoric about how such acts are unforgivable and how both groups destroy the moral foundations of the family and/or molest kids. Which seems a bit rich coming from a sex offender.
Finally, I just wonder how many individuals across the world have been prompted into experiencing intense self-reflection and guilt after these high profile celebrity cases of abuse. It seems to be a common thing for us folks with diagnosed OCD (an interestingly high proportion of people here in Remorse, including myself). Personally speaking it was partly the massive media attention given to the hideous crimes of Jimmy Saville and then Prince Andrew (still alleged... but there is an intimate photo with himself and the victim, which seems pretty incriminating to me) that made me feel particularly horrible about having had an intimate and sexual relationship with a 16/17-year-old child when I was 21 and made me think more about the power imbalance and re-frame myself as a child molester. It's kinda funny and sad how similar humans can be because so often it seems that we have similar narratives in this sub-forum or at least came to this position of intense self-loathing and remorse partly due to reading things in the news.
Anyway, I don't think you have anything to feel guilty about and I hope you can move through this through therapy.

All the best.