Since When Was Rosie a Tomboy?
TL;DR: No.
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L;R:
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Don't worry. I'm not asking to be rude or sarcastic. I'm being genuine here.
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For context, I had sort of "aged out" of Thomas around the HiT era, mostly keeping up with it through my brother, who likewise "aged out" around Season 15 or so. We also found Thomas in CGI to look uncanny and creepy, and still do, so make of that what you will. Either way. I haven't gone back to ever properly rewatch what I missed or don't remember.
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But I do remember Rosie. That said, I don't remember the fandom's assertion that she's a tomboy being reflected in the show. For those who may not know or need a refresher, a tomboy is defined as a girl or young woman who exhibits behaviors and traits typically associated with boys or men in a given culture, such as wearing androgynous or unfeminine clothing and engaging in activities traditionally considered masculine. In the case of humans, this becomes harder to define as what we consider to be masculine vs. feminine changes with the culture, but it's still usually easy enough. If she'd rather learn how to play baseball before playing through Metal Gear Rising and marathoning Black Clover instead of learning to sew before playing with Barbie and marathoning Sister Wives, she's a tomboy. If she'd rather dress in sweats or jeans and a tank top or T-shirt instead of skirts and blouses or dresses, and if those clothes are darker cooler colors instead of bright pastels, she's a tomboy. Etc etc.
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Regarding Rosie, it's harder to define. Because she's a locomotive, so she can't be judged by our standards. What she does and how she acts is limited by being a train. But even then, the franchise never really tries to assign distinctions between gender roles or gender interests. In the books, most girls were either coaches or diesels that take passengers (like Pip and Emma), so is taking passengers feminine and are girl engines that take freight tomboys? Nope. Boy engines make passengers and freight all the time, and Mavis in the books is pretty feminine, but she works with trucks exclusively. By extension, Mavis is also a shunter, so shunting isn't the mark of a tomboyish girl engine either. And this is without getting into the TV show and all of its engines being treated as mixed traffic engines that do similar work and have fairly diverse liveries.
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So what's left? Well, her personality! Which is... What? Being energetic, enjoying races, being feisty, being determined, having a sense of humor... To be honest, I don't really consider any of that to be uniquely masculine personality quirks that make Rosie a tomboy. From my human perspective, I have met many girls and women in my life with those traits, and I would NOT say that all of them are tomboys. I consider MYSELF a tomboy, and I'm not energetic or a fan of racing. And amongst the engines, I think those traits are fairly spread out across characters of both gender identities. Especially in the HiT and Nitrogen eras, where personalities were incredibly flexible so any character could fit any story. If anything, I'd say Rosie is a mother hen type for worrying about James's brakes in "Fastest Red Engine on Sodor", or a girly girl for her having an obsession with romance in that later episode. But I'm sure I'll get pushback for that.
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Ultimately, I don't think Rosie is a tomboy because I don't think she has enough of a personality to define her as one. Her sparse appearances, fairly generic existing traits, and the show not really assigning tasks based on gender identity makes it too hard to define her as anything at all. Maybe the new show will fix that and this will age poorly. But for now? I stand by this.