I think a lot of appropriation talk ignores that much of it started from businesses and governments taking cultural things wholesale and selling them on out of context for their own pockets to the detriment of others. It's only recently that it's gone into people's private homes.
For me (as a Mestizo who celebrates Día de Muertos), it's cultural appreciation is being invited to do something and taking part, which is something a lot of us do. Before lockdown, my DD1 shared in her Spanish class a common video used to explain and invite families to take part.
Appropriation is when someone is gaining money/reputation/respect for something that people within the group are often marginalized, misunderstood, and you get more credit for it than those it originates with. So, no one really gets that for jeans, but lots of people misunderstand Día de Muertos - and some of the worst are the ones trying to 'protect' it - worst I've had a couple years back is a White Brit who overheard me tell a mutual friend that I'd been so happy to find a tinsel skull decoration in the shops, said person went on a rant about how much it angers them to hear people mix up Halloween and Día de Muertos and then tried to explain it at me. When I started to correct that it's only at this time of year because of colonization and used to last a month around August with different tradition,, some which have been lost, they then tried to tell me oh yes, because it was originally a harvest festival (no it's fucking not and that doesn't even make sense for Central and Southern Mexico which has a very different climate to the UK, but this person felt really confident pulling that out of their ass).
Reminding people that the Bond movie with Día de Muertos parade was the first time Mexico City had one and they kept it because they liked it so much. It wasn't really seen as universally Mexican until fairly recently, Mexico has a lot of different cultural groups and many are mash-ups of each other and others.
So if you want to wear skeleton costumes/makeup, or make an altar with photos and marigolds and offerings, and discuss the dead and your thoughts on death, decorate graves, go for it. Photos for your private collection, great. If you then feel the need to post it online as many internet personalities do to get recognition for doing this, I'd ask you to consider why you feel the need for that recognition/benefit from your appreciation and that you can't be surprised with the internet as it is that some will get mad. I likely wouldn't unless it was a real amalgamation of Celtic Halloween (with the dead being scary, very much separate from us) with Día de Muertos (dead being honoured, recognizing it's what we'll all become), but I wouldn't view it as appreciation either.