It is if you keep the nutrients the same. Just seeds and veggies are still not enough, as you need to find the right kind of foods to cover all of the nutrients a bird needs, and there's no magical vegetable that has all of it. That is why pellets can be good, even if you don't give it to them every single day.
Besides. We, as humans, don't have the same meals every day, and unless if you're really careful about taking in all of your required nutrients daily, that means you don't usually have a balanced diet, either.
Provide it balanced when it's possible. If that is the best thing you can do, then go ahead and do it.
It's also possible to buy pellets online in large packages for less, so if you can, that is also an alternative to consider.
