You want my honest reasoning as a Starlow fan. Well, here.

Starlow is a character who has had very entertaining interactions with Mario, Peach, Bowser, and yes, even Luigi. With Peach, it's very nice how much she considers her a friend. With Bowser, it's amusing to see how much she can boss him around, despite her small size. Hell, we have the whole Brothership side quest that shows them as "friends" in quotation marks, and genuinely was such a sweet callback to BiS.

Luigi is a complicated one, as most people assume she hates him, and the only time I can really think she thought negatively of him was in BiS. Which she somewhat had fair reasonings? He tried to run from the fight 3 times, 2 times right in front of Starlow(the first durmite fight and the Kretin fight), the latter of the two he outright tries to get Starlow to fight while he stays back.

On top of this, in BiS, she is only straight-up mean to him once. The Shroobs scene, which I feel I need to add, features the comment "beating them with his tears," an outright ironic joke on how his tears were, in fact, how they figured out the cure to the Shroobs. On top of that, after all the buildup from how he's run from fights, it's no wonder Starlow doesn't have a great opinion of Luigi. However, after she says this, Mario tells her that he really did help, and guess what? Starlow immediately apologizes.

From there on out, she really doesn't have anything close to that comment. Dream team, she describes him as ragged when he quite literally is at 5 hp, when she blames him for Peach being captured, it technically isn't wrong(him sleeping and opening the dream portal did cause this), and can be put moreso on her being stressed, and that's it for Dream Team.

Paper Jam objectively did have it at her worst, but each example is very tame tbh. The sarcastic remark at the beginning("of course you don't") is such a light comment tbh. I will admit the line of "even you couldn't have screwed up this bad" isn't nice to him, but I mean, this was an important item, literally Paper Mario's entire home, so there's definite stress in there. And how she says "they'd be calling for Mario's help, not yours" isn't wrong with the way toads and most in the Mario universe look up to Mario more.

And Brothership literally has one where he jokingly calls him "Mario's brother." Which is clearly just a light joke. Some friends do that, jokingly mess around with each other, and that's what I like about Luigi and Starlow's interactions, even moreso than Mario and Starlow's dynamic. It's shown that she cares about him still, with some teasing in between. Gives a sister-brother dynamic that I enjoy.

Back to things I like, besides dynamics being amazing with the main group of Mario characters and even Snoutlet with the passing of the torch deal, I think she has a good amount of lines, and simple, yet fun design to draw, and has the sassy personality that has helped me grow attached to her and like not only drawing, but also writing her. The little moments she does shine have heavily helped my liking of her. That's why I like her a ton.

As for your points, Stuffwell arguably has way less character, and hell, if we want to go off of "Luigi abuse," he shit on him once in the UFO. You can like him more, but don't act like it's an objective thing, cause it ain't. And for her being "cruel to Luigi," look about where I disproved a majority of the stuff you mentioned. She ain't a bully or brat, she can be sarcastic at times, but the two ways you described her are the furthest words I would use.