No, what's ironic is that you don't realize the two are actually the same thing.
You cannot have one without the other, how can one improve if they don't know where improvement starts?
Self improvement and party evaluation are not mutually exclusive concepts, they go hand in hand.
Finding the weakest link in the party also means that player now has a point of reference to start improving, any player can sit there and spike high numbers on a stationary dummy parse because it doesn't move or hit back, the real test is going into live content where it matters and seeing if your skills and gear hold up while you deal with fight mechanics.
Maybe that player learns they need to hold a GCD between a certain move? Maybe that DRG knows they are going to lose BoTD due to a phase change so they need to adjust before hand? Maybe the BLM has to change the timing of when they toss down Ley Lines in order to get the most out of it's duration? Maybe your accuracy was too low for the encounter or maybe you're overshooting by 100 points and can swap out a piece of gear?
These are things you learn in live content and are exactly how you improve your play.
The thing is this happens at the same time you are also trying to clear content and this is where proper evaluation and data helps and if a single player isn't up to the level of the content it doesn't mean that the 7 others in the party aren't either, they should have every right to know this and adjust accordingly if they need to in order to continue making attempts on the fight.
The key is not taking critique as a negative.
It's entirely possible to be the weakest link one run, get some practice in and turn around to top the charts the next time around, but if you had no idea that you were under-performing before then how would you have any idea whether or not you needed to improve?