What mythologies have poor representation in media, in your opinion?

Pretty much all of African mythology. Sometimes it gets mentioned tangentially or included in a half-assed way, and even then it's almost always misrepresented. Anansi was my favorite character in American Gods, in part because I see so little African mythology, but also just because he's badass.

Zoroastrianism would be cool. Basically the answer is anything not europe or specifically egypt but I always felt that for example a hindu god killer game would be fkn sick. Like either GoW format or Diablo-like.
- Well even in europe there is much beyond greco-roman and norse, but that doesn't get any media coverage
  
  That's true, there is some really cool stuff that isbrarely mentioned.

Indigenous people of north america. Tokenized parts of tales and folklore are often misrepresented and not told by people who grew up learning the stories.
- And, they're treated as a monolithic mythos, as though the myriad peoples all had the same, exact set of beliefs and entities. It's a mess
  
  I'm sure this also happens in Australia with our indigenous but I don't interact with them enough to observe directly.

All mythologies except maybe christian mythology have very bad representation on mainstream media.

Look at symbols of Christian holidays, then read where they came from. I'd say that represents misrepresented mythologies. It's all stolen symbolism to blot out the competition.
The Easter bunny giving eggs? Spring is about fucking and both of those are symbols of fertility.

If by poor representation you mean misrepresentation then Islam hands down. If you meant lack of representation then I'd say zoroastrianism and celtic paganism. There are plenty others I'd love to see but I picked what I did because we know enough about them with enough certainty on details that we could accurately represent them if we tried. I'd also love to see Bedouin paganism and Mayan mythology accurately represented in popular media but this is much more difficult to accomplish.

Greco-Roman and Norse mythology are represented a lot, but in a very shitty manner with barely any research, let alone respect.
- Yeah it's funny that those were called out as exceptions because as far as ancient Greek mythology goes, Disney's Hercules (one of the most widely known takes) basically made Hades = Satan and Zeus = God. I've also seen Hades get treated as Satan lite in DC Comics adaptations, basically unashamedly rewriting another mythology to base it around christianity. Meanwhile, from what I've seen, Hades was actually one of the few semi decent gods.

I'd really love to see more representation from various indigenous groups and nations in the US and Canada, like stories and myths from the Anishinaabe people being made into a TV show or animated movie or something (just an example).
I think an anthology series (either one overarching story per season or one shorter story per episode) would be really interesting and probably also allow a lot more indigenous people to break into acting/production. Using music made by indigenous people would also be cool.

Not quite on topic but I think it's crazy that Smite features the gods of extant religions like Hinduism but doesn't have a character that transformers between The Father, Son and Holy Ghost with unique move sets for each
- It's the only game that features a sigificant amount of pantheons for me. All others keep using overused pantheons!

Always cool seeing Yoruba stuff in media. Just watched the new season of the Castlevania spin off and it features a few different pantheons; Aztec, Yoruba, Voodoo, Egyptian. Ogun was badass and loved him telling his story.

There are lots of adaptations of Greek myth, but none of them are especially faithful to the source material

Every single one. They ethier white wash (not in the making people white way but sometimes they too) or villify everything about it. There is no place for nuance in modern retellings for some reason.

Japanese and Chinese myths and legends have excellent representation in games and movies: the Egyptians have representation and followers everywhere! The Celts and Germanic peoples contributed pretty much everything found in European fairy tales. The Middle East gave us their myths and their gods, and people from European/North American cultures know at least a few Hindu Gods and their tales, again, often thanks to video games. That leaves sub-Saharan Africa, Southeast Asia, Australia and Oceania, and the Slavic and Siberian myths 'underrepresented'. Can't say about middle and South American ones: I suspect they are better known in the Americas than in Europe, but I dislike them, so haven't the background to be sure.

Christianity is virtually always represented by those who either don't understand it, or are deliberately slandering it.
- They're deliberately talking shit about it because they understand it, and its history. It's not slander if it's true.
  
  If your understanding of Christianity is based upon how well mainstream Christianity understands it, then you don't actually understand it, only the warped form that it takes after thousands of years of infighting and political manoeuvring. For example, did you know that at one point it was actually taboo to depict Christ (or Mary) as a human?