I see rollers pretty much all the time. Hordes of Krak-On's everywhere, a fair mix of splat rollers, and a fair number of carbon rollers. And the obligatory S+ ranked Dynamo every 5th match.

I'd say there's at least 1 roller in about 60% of all matches I play. And some of them have 5. But, then I predominantly play Japanese players to the point that most of my lobbies are 7 Japanese players + me, so maybe that's why. I'm not sure I saw as many during Splatfest, though I get matched largely with higher rank players and so I saw a number of Dynamos.
I've pointed out a few times that the matchmaker seems to do weird things with grouping up multiple of the same weapon, or identifying what weapons you main. When I take a roller I see tons of rollers, when I take eliter I see tons of eliters. It's possible that some people that play mostly shooters are matched mostly with other shooters and thus see less rollers.
The nerf is almost certainly related though. It took what was a low skill floor weapon and turned it into a very high skill floor weapon so many understandably moved on to greener pastures. Not everyone is a masochist who seeks ONLY the high skill floor weapons like me ;)
It might be a bit incorrect to say that Westerners are "lazy" in their games compared to Japan. I think the West, and especially Nintendo players have more "casual" players who aren't interested in high challenge that still play on consoles & handhelds, while in Japan the casual users have moved squarely to mobile. The same "lazy" casual players exist in both places, it's just that they mostly play on a different platform in Japan. In the US a home console is still a family item of interest to anyone (it's on TV!) while handhelds are something for very dedicated game lovers (and, yes, kids.) In Japan I think if you buy a home console it's because you're VERY into your games, while handhelds and especially mobile attract that wider more casual pool. So it's a self sorting system. Japanese WiiU owners are going to be mostly "core gamers" who want challenge. Western WiiU owners are going to be a mix of "core gamers" and a lot of recreational gamers. I'm not sure it's really much of a cultural difference in this case that contributes - serious players are serious in both places. Though I still feel the Japanese serious players approach the game with a different, more friendly attitude while Western serious players are "all business."
I've seen that too. I posted in the salt thread about that yesterday actually! It's mostly going to be carbons doing that, I haven't seen many splat/krak-ons doing that. I'm not sure if it's people that have simply adopted a (cheesy, cheap, dishonorable) camping strategy (that's, in my book, right next to spawn camping in the "total ahole" book of plays) with carbon rollers, or if there's more and more brush users trying out carbon rollers. I expect brushes to camp. I hate them for it, and loathe having to deal with them, but I understand due to their otherwise limited abilities, that's what brushes need to do. But rollers are just cheap if they do that.
Also of interest is that I don't think I've seen any Japanese players do the cheesy camping - it's always western players. And as a carbon main myself, I don't play it that way. I may lie in wait for 5-10 seconds or so if I see an enemy may be coming my way (which is what you have to do) and then continue rolling/flicking along, but I don't sit in their own base in random splotches of ink for upward to 40 seconds of a 3:00 TW match waiting for someone to kill. If I wanted to do that I'd just have brought eliter. If I'm done painting new ground, I might lurk around mid underwater like a shark - but that's mid, deep in our own ink, and they (should) know I'm there, not teeny tiny splotches on ledges within enemy ink. But, then, I mostly play the Japanese, so I've adopted mostly Japanese playstyles. :)
The camping strategy is kind of a "junior spawncamper" strategy. They don't sit at your spawn in force. They hide in random, undetectable, unflushable (short of echo/PS) locations but next to high traffic areas they know you will enter, WITHIN your base area. It SHOULD be a losing strategy in TW since a roller should be a main contributor to your team's turf coverage and recovery from failed pushes, but somehow it seems to win, because nobody ever knows which base exist is clear, and causes both hesitation and likely splatting on attempt to exit.