I feel odd because Long haired Lars? People say he's a trans man but now that's he's pink(a social concept of a female color) and they have him with long hair (typically to make someone feel more feminine) wouldn't that just cancel his feelings?
First, I’d say that the length of his hair has stayed pretty consistent throughout the series.
The poof is a little bigger but the art style itself has changed since the first season, so I think it’s meant to be taken the same way. So I don’t think the length of his hair is too much an issue.
Second, I’m going to say immediately that I don’t subscribe to the transgender Lars theory, because I, personally, don’t read his character in that way based on the things we’ve seen about him. Regardless, I’m not going to judge whether the theory is wrong or invalid, because there are multiple ways a character can be read and if people identify with him in that manner then far be it for someone like me to tell someone how they should enjoy the show.
But working in that paradigm, I would then ask what Lars thinks is masculine or feminine, and whether that affects his presentation of gender. Because as you note, pink is “typically” construed as feminine (though I think less so over the years), but there was a period is was a masculine colour. Besides the change of trends over time, the idea of masculinity and femininity also varies in different places over different cultures.
I think it’s rather clear in the show that the characters come from a variety of cultures as well. And even beyond culture, there are individual preferences. There are men who feel masculine but sport long hair and wear pink. So Lars’ own preferences should be considered as well.
And from what we’ve seen, Lars doesn’t seem to have a problem with pink. The still above is from Future Boy Zoltron. The same pink shirt appears in The New Lars (but it could’ve just been Steven dressing Lars, as he wore the ear plugs that the latter’s mother got him, but Lars himself never wore). Here he is though, wearing pink, walking around town with Sadie.
So I think that the pinkness of his new colour scheme isn’t going to be a huge issue if you’re considering that theory.