The value of the 80/20 rule is that it reminds you to focus on the 20% that matters. You should identify and focus on these things. So in bodybuilding, what are they? I would say that the 20% that matters includes:
Researching & following a good, fundamental, bodybuilding program. (Not a perfect one, it doesn't exist).
Putting in hard work in the gym, consistently, over a long period of time.
Following the rule of progression, and ensure that over time you are lifting more weight, more reps, or more sets.
Having good nutrition. Eating enough good stuff, and not too much bad stuff over the course of a day.
Getting adequate recovery.
Adjusting your plan periodically, based on your results and your experiences.
Which basically means: Train. Eat. Rest. Repeat. Week in and week out. Focusing on the basics will give you 80% of your results.
So if that's the important 20%, what's the 80% that's trivial? Well in my opinion it's details like these:
Should I do 3 sets of 8 reps or 5 sets of 10 reps?
What's better, 1.25g protein per pound or 1.37g/lb, or 1.5 g/lb.?
I'm doing BB curls, should I be doing DB curls or EZ bar curls instead?
What's the best angle for incline barbell presses?
If I don't get 30g of protein within half an hour after training, is my session wasted?
How much should I be lifting for my height / weight?
Are DB flyes better than using the Pec Dec?
Etc. Etc. Etc. Honestly, that stuff doesn't make a difference. Or rather, if it does it makes a relatively small difference (20%); or only makes a difference for a relatively small few who are at the limits of their physical development. For most of us average Joes, it just doesn't matter! Sure, if you have a wrist issue, EZ curls may bump into that 20% of things that matter, but in general it doesn't make that much of a difference.
The muscle mags would like to convince you different, since they want you to continuously tune in to find out if you're doing everything right. If you're "in the know" about the latest "hollywood workout". Whether you're missing a miracle supplement that just got invented in a secret Swiss lab.