Personally, I would not feed mealworms as a treat unless I was raising them myself, just because they're so very expensive! If I wanted to add protein, I'd start with any excess eggs; table scraps (meat, eggs, fish, cooked beans); and access to a compost pile (bugs! worms!)
If I was going to buy a protein supplement, I'd look first at Gamebird starter or Turkey starter (unmedicated only), then at cat food or kitten food (kitten food has higher protein than stuff meant for adult cats.)
My reasons mostly have to do with cost and convenience. Table scraps, extra eggs, and compost are things I don't have to specifically buy (saves money, saves time and effort). The gamebird starter is intended to be fed to a similar kind of birds, is often around 24% protein (which is more than most chickens need), and is far cheaper than the other things I could purchase. The kitten food costs several times as much, and runs about 40% protein. The mealworms (dried) cost more than FIVE TIMES as much as the kitten food, and are 50-something percent protein. So the mealworms just look like a rip-off to me.