[This Anon is referring to this recent reblogged post]
Valid question! No, I don’t see it as being rude to show appreciation to fanart by reblogging it and tagging it w/ Tumblrand Hyperbole™
as follows:
“#stealing this for my fandom”
That was the tag I came up with when I first started wanting to reblog fanart from other fandoms (or original characters) that could also pass for fanart in our fandom, so that I could keep it separated from the “#fanart” tag I try to keep purely VC. I don’t think I even use it that often.
It’s Tumblrand Hyperbole™ for me to say I’m “stealing” it. I’m not literally stealing anything; when I reblog things like that, I don’t even add visible comments like, “OMG THIS LOOKS LIKE ARMAND” or anything like that, which could be considered kinda rude. When people then reblog it from me, they can add comments and/or tag it however they want (in fact, there are people who do reblog these kinds of posts and original posts from me and add visible comments like, “THIS IS SO [X CHARACTER IN OTHER FANDOM]”) and I don’t mind at all, bc this is a social media platform and people like to add comments to engage w/ others here.
Whatever my tags or reasons for reblogging something, it’s still true that Reblogging gets that fanart more attention than a Like. I’m still promoting that fanartist. Especially when I’m NOT adding any visible commentary that would detract from their artwork.
Side note: I have never received a message from any active fanartist asking me why I tagged one of their works "#stealing this for my fandom,“ and I would happily remove the tag upon request.
What if someone sees the fanart bc I reblogged it and decides to follow that fanartist? The fanartist gains a new follower! Yay! What if that fanartist also offers commissions? I’m advertising their work! And maybe they’ll be willing to draw fanart for our fandom, since the character we would want might be similar to something they’ve already drawn.
There are many similar-looking characters out there, just as there are many similar-looking people out there. Francoise Brunelle did a series of strangers who look like twins. Here’s one example (which I picked bc they’re cute, but there are more identical pairs to choose from) [X]:

There are also websites like https://twinstrangers.net/, and http://www.ilooklikeyou.com/. I’m sure there are more.
TL;DR: What I’m doing when I’m ”#stealing this for my fandom" is actually remarking on the uncanny resemblance that character(s) has to ours. There are plenty of auburn-haired teens in all kinds of media. If I choose to reblog fanart of one of them, it’s out of appreciation and I’m still promoting that fanart.