The stringy parts on celery are not recommended, as they cannot be digested and can lead to intestinal blockages. Many people advise against giving celery at all--- especially because it is fairly empty in terms of nutritional value and it can fill your bird up (leading to deficiencies)---Ice burg lettuce has a similarly filling effect (without much nutrition).
Peanut butter is also not something you should give your bird (except in TEENY TINY amounts (like, as a treat once in a while)...1) It is high fat. 2) Most are high in sugar, 3) it contains salt, 4) many contain preservatives.
If you feed the vegetables, corn is sort of a filler (high in carbs, low in nutrition)-- you might consider removing some of it. Eating corn on its own corn isn't deadly for any toxic reason---it is deadly in the way that a truck-load of cookies and ice-cream could be deadly to a child (if that child were allowed to choose what he/she ate). If you gave most kids a plate of steamed broccoli at the same time that you gave them a plate of cookies and said "there you go! Eat whatever you want from there", most kids wouldn't touch the broccoli and would only eat the cookies. Do this for long enough and you will have an obese, diabetic child with nutrition deficiencies and behavioral issues. Your bird is similar, in that it gets to "pick" what it eats within the mix and the choices made may not be the healthiest--the logic then being that you should consider removing the unhealthy option (or at least some of it).
I wouldn't feed veggies still frozen, but I don't think this mix would be a problem as long as your are mindful of the fact that your bird may ONLY eat the corn (problematic)--The best way to thaw veggies without cooking them would probably be to soak some in hot water and then strain (if you want them to stay raw). FYI- To the best of my knowledge, all of the vegetables in your mix are safely eaten raw, but keep in mind that some vegetables need to be cooked in order to be safer for birds.
Side-note: As long as the veggies and fruits in question are safe to eat raw (for a bird) then keeping them raw will preserve more of the nutritional value. That having been said, some birds prefer the texture of cooked veggies and in that case, experiment (just beware of hot-spots if using a microwave).
**Make sure you check your box of pasta before feeding it to your birds---I like to give cooked pasta (a noodle) to mine as a treat now and then, but the vast majority of pasta sold at the store is fortified with added vitamins and mineral--this is not the type that you want to give to your bird, as these vitamins are added with humans in mind, and can overdose a bird- fortified cereals, pastas, juices etc should not be fed to birds**
I feed mine "Rozoni" (brand) "Gluten Free" Rotini, as it was one of the few pastas I could find at Walmart that didn't have added iron etc (I am not a gluten-free person).
As far as I know, my brand seems safe-ish (at least as an occasional treat).
Ingredients-white rice flour, brown rice flour, corn flower, quinoa flour, mono and diglycerides.
Here is the nutritional breakdown listed on the box- Vitamin D: 0mcg, calcium: 0mg, Iron: 0mg, Potassium: 65 mg (2% of a human's dietary needs).
200 calories per 3/4 cup, 1.5 g fat and 4 g protein.
Bottom line: if you see a bunch of vitamins on the side of your pasta, they are added and should be avoided.