Screen Brightness on Replaced Screen From iFixit

That's absolutely, yet unfortunately, normal.

There are varying quality levels in replacement screens and when you have aftermarket LCD's, they will typically be a bit dimmer and have shallower viewing angles. Lower cost screens will also have lower quality glass which can affect the touch sensitivity and make them easier to crack again. The biggest problem with low cost screens is cheap flexes, cheap frames and poor quality adhesive to hold it all together. They actually use hot melt glue some times or even worse, just double-sided adhesive.

iFixit tends to sell good quality screens and they even have a "Choice" line on some models that feature cold-pressed frames and original-quality LCD's among other things. The problem in the industry right now is that customers believe they are getting Original Apple screens when they pay someone 60$ to replace a screen. They don't understand that there are quality levels so everyone pretty much has to sell the cheap stuff to stay competitive.