Is my cat too skinny?

She's been to the vet this month for one year checkup and to a spay and neuter clinic to be spayed. The vet and vet tech said she was small but not that she needed to gain weight or that they were worried about her.
She was 6.25lbs at the beginning of the month, 6.13lbs at the vet, and 5lbs(though I didn't confirm that this was not a typo and that they didn't mean 6lbs) this week at the spay clinic. I've always been able to feel her hips and ribs easily with little to no fat cover. She eats about 200-230 calories a day.
Looking down on her:

Side before she turned all the way around:

She has some loose skin on her belly but has been dewormed regularly.

Loose skin on the belly isn't uncommon and doesn't have anything to do with being over/underweight. Its just a genetic trait some cats share with big cats in the wild for unknown reasons (there are guesses out there but I've never seen a solid "this is why" thing). My Link has a pouch and Rocket doesn't. 

Sounds like she is just a small kitty. My guess would be that she'd still growing and will fill out a little more as she gets older. A better judge of over/under weight is if you can see a "waist" and you should be able to feel ribs under a layer of fat. I wouldn't worry that she doesn't have that layer of fat at this point because she is still young and could still be growing. My Mom's cat Nightfury didn't fill out and get that fat until almost 2 years old; until then he looked like a little stick kitty but he was healthy and we didn't change anything. He just stopped growing and filled out. 

Rocket is a little kitty too. She's tiny compared to my other two and usually is in the mid 6lb range at the vets but she has a little round body just dainty small overall. I would say that the 5lb is a typo at her spay because that would be a HUGE drop in a kitty her size (imagine a normal size adult dropping about 30 pounds) and you would have noticed it. And if she really went from 6.25 to 5 in a month you'd have some health concerns to look at. The tech might have dropped the decimals when they did her intake since if she was fasting she might have dipped slightly below the 6lb mark. Rocket is on the one on the pillow in this photo, you can see she's just small. 

If the Vet didn't say she needed to gain weight, then i'd say she is just perfect   They will normally tell you one way or the other.  (usually it's the other )

Maybe she has a higher metabolism. I'm convinced that cats are a lot like people in that some can have a faster metabolism and others can have a slower one. One of my cats shows signs of a high metabolism because he needs a lot of carbs to keep on weight, and wet food alone keeps him very thin. Meanwhile his brother gains weight very easily and needs a diet in low to no carbs. If her weight bothers you try introducing more carbs into her diet (mixing kibble in with her wet food) and see if that makes a different. Or small snacks throughout the day.

If she's a younger girl she'll have an easier time adapting to a new diet too