Off-topic implies the posts tagged under it don't belong on Danbooru. Using it for western-style images indiscriminately is wrong and invites flags from people who think flagging anything that is not anime is ok.
Posted under Tags
This is exactly what I was afraid of with this tag. I really don't want to see people tagging this on approvals they don't agree with. If you think a post is truly off-topic, you can flag it for deletion and let the modqueue decide. If it gets deleted then you can tag it as off-topic. Otherwise leave it alone. I don't want to see people edit warring over this tag, or using it to passive-aggressively tag things instead of flagging them because they know an actual flag would never go through.
I agree with what Renim said in that if a post is approved by the modqueue and is generally accepted by the community, then it's more or less on-topic by default. Sometimes the modqueue gets it wrong and sometimes the community may be divided over a post (see: the Doom posts), but in general, if something isn't strictly anime-related but the community at large accepts it, then it's de facto on-topic. Something like post #3531198 might not be strictly anime-related, but if community finds it acceptable then I'm not interested in going against what the community wants.
If we want a tag for western art, that's something we could discuss, but it shouldn't be this tag. Calling a post off-topic is begging to start an argument unless it's very clearly off-topic. I can see the desire to have a tag for western art, but it's something I can already see would be fraught with difficulties.