I believe a spoiler tag would be great
I'd definitely encourage spoiler tags for Animal Crossing Bells Time Traveling as even though not everyone considers stuff like that a spoiler, it matters more if somebody who does sees things they do not need to than it will if individuals who do not just need to look at something which's already been spoiler tagged.
In terms of memorials, I believe it'd definitely be best to restrict them somehow. While they do not both me in the smallest and obviously people who place them don't mean them to be upsetting, you sort of have to cater to the minority because these images could be very upsetting for some people so it is far better to have some precautions.
I would also support what another commenter has stated about flaring edited screenshots. It would just be good for transparency and anyone new to the game will not think that they can do things that actually are not possible and be let down.
I believe a spoiler tag would be great, especially since we're approaching one year at the game. People have already begun posting stuff about how dreadful the eggs in Buy Nook Miles Ticket bunny afternoon were, and it is not reasonable to newer players who havent seen bunny day (or whatever thing) yet.