I agree with the idea that attitude and behavior are greater defining characteristics than looks, but not necessarily with liking stereotypical "manly" things, although the attitude can often incline one towards liking those things.
At least when it comes to real life as opposed to fiction, I think what defines a "tomboy" is the fact that there's a bunch of incomprehensible bullshit that women tend to obsess over that tomboys generally don't care or get hung up about, which is what makes them more relatable. Basically, they're uninterested in the more frivolous aspects of femininity.
I feel like in fiction in order to convey certain characteristics it can help to have a character be more defined by certain activities. Like enjoying cooking is something I feel like is often portrayed as a feminine activity in cartoons. Personally, I do way more cooking than all the women in my family, in part because I pump a lot of iron and being in control of your nutrition is vital in seeing any results from that, and I'm sure most guys who take strength training seriously also spend a lot of time in the kitchen.
Shows (and real life) sometimes try to make a cool girl who's "one of the guys" by just making her like video games or something, but that alone doesn't make a girl tomboyish. I have a coworker who's a fake gamer girl who tries, and claims, to be this, but she's probably the most feminine person I've ever met (and not in a good way--gossip, pettiness, tons of make-up, designer purses she can't afford but can't help buying anyway, etc)
On that note, I kinda felt like Babs from The Batman could count as a tomboy? There's nothing overtly masculine in her interests aside from superheroing--and in one episode as a plot device, strength training--but she doesn't spend a lot of time being "girly," and in some eps she comes across as kind of a dork regarding her enthusiasm for being a hero. The sporty look, while not vital, kinda adds to the feel.