My thought on this:
-The first graph shows that the phenomena have be happening across the board. And from where the data started on LGBT group it is a 2.6 difference between the increase and the LGBTQ category and the entire sample. You didn't link to the study, so I don't know the margin of error, but 3 is generally seen as a pretty love one for things like these. So it could be the difference between the two groups could be statistically insignificant. This would lead the only takeaway being that all high school students are feeling more unhappy.
-In fact, women seem to have the highest increase from the LGBTQ starting point. A lot of positive things have happened with women's rights during that time
-LGBT category is a broad category so we don't know how it is broken out. Given the sample, what could be driving this might be the feelings of transgender kids or just homosexual kids. And I would think locations matter. Being gay in affluent blue suburbs and urban areas is probably different than doing that is suburban Utad and rural America. Not saying this to hand wave, just that things matter generally.
-I guess the second part is supposed to say that given all this happening, you would expect a different outcome. But High school students have a very short reference point for progress than an adult.
If I would guess, social media has a big role to play. That while it can magnify good things, it also does the same with negative things. So LGBT kids might be getting exposed to a disproportionately high amount of hostility online relative to the general public. Also, they are in high school, social media makes cyberbullying much easier. So yeah, they have it easier as far as how bad previous generations had it on the civil rights front. I would agree, but there are things much different than the world Gen-Xers, most millennials, grew up in; the media students consumer. That could play a part.
Like I said, the shift among girls seems to be higher too (again could not be in reality). Women have had some high-profile culture wins over this period as well, and they still feel ****ty. So this lack of uniqueness makes me less convinced that we should see a different outcome with the LGBT group.
I mean just guess here, but if someone said that the growing hostilities toward LGBT kids in some areas of society is landing harder on kids than the progress made in other areas. On aggregate society is progressing, but people feel worst being the realities of a high school kid are unique.
A major thing is Behavioral Econ is about this. That is what is salient to people motivates their feelings and behaviors even though rationally you would expect a different outcome.