I need tips on how to not look like an absolute asshole when giving constructive criticism to others. What if I see some post that has some mistakes (e.g. weird anatomy, character with too many neon colours, etc.), and I give some tips to the artist to help them improve, but some person mistakes me as being rude?

Try complimenting what's good while pointing out a flaw. "You did great on the hands, but the shoulder's a bit pointy." People can't tell you to use a blacklist over something like that.

Also it's worth pointing out that 99% of posts are uploaded by somebody else, and not the artist.

pup said:
Also it's worth pointing out that 99% of posts are uploaded by somebody else, and not the artist.

Well, yeah, I know that, but what if the uploader made the art?

I would highly recommend also ensuring first trying to figure out what the intention of the artist was with all the bits and pieces and then go from there. Many people I see that are trying to critique things and try to point out mistakes are done deliberately for some reason or another, and critiquing them as if they were accidents or not set in stone is usually a quick way to get the artist to not like you, or dismiss your input entirely.

One famous case in point for this is a "character with too many neon colours". If the character is from a paying costumer of the artist the color scheme used is not something the artist can pick, and thus telling the artist the color scheme is bad is just entirely worthless information and a waste of time for both of you.