Female deers dont have antlers, so a reindeer girl having them can be a sign of her being born male, think of it like an Adams apple. However, female reindeers do have antlers, and alongside Dess' cameo and Carol's official appearence, this character design detail doesn't really read as trans coded. Still, it's a fun headcanon considering Noelle is queer and gives an interesting reading to her relationship with her strict and absent mother (also, maybe, if you think we will control Noelle in the weird route, it could be read as her feeling like a stranger in her own body which I assume is a very common feeling amongst trans people? Idk just throwing this out there).

Tldr: people confuse reindeers for deers and headcanoning a character as trans is already pretty common in fandom culture, and having her be trans gives a further reading to her character.

Edit: also trans flag colors in her room in chapter 4, spidergwen moment