It always seems pretty popular to start the 90s around 1992 or around the fall of the Berlin wall but, looking back at 1991 it seems very in between the 80s and 90s. A lot of the music that year doesn't sound 80s but, many of the music trends that would define the 90s hadn't fully arrived yet in 1991, this will include a lot of things outside of music too.

More like the 80s for the most part. It wasn't until Christmas of 1991 when the USSR collapsed, and the biggest song of the year was Bryan Adams "(Everything I Do) I Do It For You", it wasn't until 1992 when "Smells Like Teen Spirit" peaked on the billboard.

It was the '90s, just not really the same '90s as the core of the decade. It's '90s by definition, so everything that came out counts, even if you could stylistically imagine it being released in 1988 or something.

1990 was such a stand-out year musically - Madonna's Vogue, Vanilla Ice, MC Hammer. It really set it apart from the 80s - and even 1989. For that reason alone I'm finding it hard to say that 1991 had that 80s feel. Obviously we hadn't got to the Seattle band years, but don't forget that was also very much a US thing. Other parts of the world - like the UK - were making the 90s their own. The "Madchester" scene in the UK was already making underground Indie / hippie music in 1990 (our version of grunge). It was a  far cry from anything 80s.

I would say 1991 was more 80s. In showbiz, it was more 90s.

Check this link:
https://www.tostepharmd.net/kermanphoto/schools/kuhs1991.html

I'd say 80's, but barely.

1992 and especially 1993, on the other hand, were absolutely more 90's than 80's

In the U.S. at least the pop music charts were almost a 50/50 split of a more '80s style versus what we came to see as '90s. Alt rock really took off that year in the underground scene though, with the release of Nevermind, Ten, and the first Lollapalooza.

In the summer for example you had this in the top 10

http://www.youtube.com/watch?v=4Fc67yQsPqQ

at the same time as this

http://www.youtube.com/watch?v=kcYIZ-cduvM

It depends on what aspect of pop culture you are talking about.

Musically -- Looking back at the Billboard Top 100 for 1989, 1990, 1991, and 1992, I would say 1991 is close to a 50-50 split between the 80's and 90's. More than half of the top songs in 1990 were still very 80's in sound, despite some of the 90's beginning to appear (65/35 split, IMO). 1992 was the opposite of 1990 -- 35% 80's, 65% 90's. 1991 was truly a "bridge" year that doesn't feel very definitive either way. At least with 1990, there seemed to be a heavier late 80s presence.