There are plenty of opportunities in the books where it would have made sense for her or others to mention that she is vegetarian, if indeed she was. This isn't to say that every vegetarian character must go around evangelizing, but Cassie frequently makes moral arguments where it would have really helped for her to bring it up.
On two occasions, aliens point out to her that humans eat animals, and she never states that she doesn't.
#9 - The Secret:
"But you eat some animals", Ax pointed out. "Cows, pigs, sheep, dogs."
"We don't eat dogs!" I said.
#19 - The Departure:
"It's what we are," she yelled back. "We're parasites, you humans are
predators. How many pigs and cows and chickens and sheep do you kill
each year to survive? You think being a predator is morally superior
to being a parasite? At least the host bodies we take remain alive. We
don't kill them, cut them into pieces, and grill them over a charcoal
fire in our backyards."
"We're not pigs," I said.
Hork-Bajir are explicitly stated to be vegetarians many, many times, so the series is not simply avoiding the topic. There are plenty of opportunities for it to have been mentioned that Cassie is also vegetarian.
As far as times when she has eaten meat (as a human), I only found one example.
In Megamorphs #2 - In The Time Of Dinosaurs, it was her idea to cook and eat the Tyrannosaurus they killed. She also instructs them on how to make the meat into jerky. The other characters are visibly surprised that Cassie is the one with this idea, but mostly they appear to be reacting to her survivalist attitude (Ax is the narrator at this point, and chiefly just notes everyone else's surprise).
This is the only time I have found where she explicitly eats meat.
In #44 - The Thing, she refuses to eat the witchetty grubs offered to her by the people she is staying with, but only because they gross her out. They remind her of Taxxons.
She does eat at Taco Bell a number of times, but it's never specified what she gets. (#16, #22, #23, & Megamorphs #4)
HOWEVER, there are two explicit occasions when Cassie specifically eats vegetarian.
In #12 - The Reaction, Rachel is alone in a hotel room, and orders a burger via room service. When Cassie calls to say she is coming by, Rachel calls room service back and orders a salad for Cassie.
In #43 - The Test, Jake brings burgers over to their table in a food court. Rachel and Jake both have burgers, and Cassie has a veggie wrap.
It's a bit notable that Cassie specifically eats vegetarian in these scenes, but the other Animorphs each have a "signature" food as well. Whenever they're not eating somewhere out of necessity, or eating with their parents, they tend towards certain foods. Rachel eats burgers, Marco eats nachos, and Ax eats cinnamon buns. Tobias mostly eats mice, and doesn't go to food courts that often. Jake just tends to eat whatever the person he's with is having.
So eating vegetarian could just be the way her character is differentiated in these scenes.
In conclusion, the character is never said to be vegetarian and never denies eating meat. Other characters are explicitly stated as vegetarian. She explicitly eats meat one time, but they had no other choice at the time. However, other than that one exception, all food she gets or eats is either vegetarian or could be vegetarian (such as "burritos" meaning bean burritos).
My subjective take is that Cassie is characterized as a "farm girl", and spelled out to not be squeamish about things relating to animals (like blood or feces). This would probably mean she also has no illusions about where meat comes from, especially since her family's business is funded by a pet food company. Once the series is underway, she probably ceased eating meat after her experiences as an Animorph. As the quote in the OP highlights, she knows that animals eat what they eat out of necessity, and so she may be choosing to respect that and no longer eat meat simply out of desire. However, being an Animorph frequently puts her in positions where she has to eat whatever is there. This would make it the same as her not calling herself a pacifist, despite her belief in it, because she often has no choice but to fight.
Lastly, an out-of-universe take on it would be Marco would absolutely have to make jokes about it if she ever said she was vegetarian. The author may have wanted to avoid having to write mockery about it, either to not ostracize young readers considering vegetarianism, or just because there's already enough ethical debates between Marco & Cassie in the series without adding that in there too.