Well I guess it all depends which definition of the word "Critical" that you use.
Cambridge dictionary has one of the definitions as "expressing an opinion about something or someone, esp. a negative opinion"
When I google "definition of Critical" the very first hit is an online dictionary that defines it as 1. Expressing adverse or disapproving comments or judgments and 2. Expressing or involving an analysis of the merits and faults of literature, music or art.
I think most of us use the word critical in the above contexts so that is where it is used in context as a "bad" thing, or more appropriately a negative thing. As to why many quilters are "their own worst critics" in that they are quick to point out their own mistakes or perceived short comings, I often think it is a sort of "defense reflex" so as not to have to endure the sting or barb of someone else pointing it out. If the maker points it out first (even though many viewers of the quilt won't even notice the perceived mistake) it is their way of letting viewers know they are well aware of the problem and there is no need to point it out as the maker already has.
There are loads of other reasons too. Being humble, avoiding pride, being self critical themselves or having lower self esteem or modesty. In the case of modesty there is also the occasional false modesty where the self criticism is actually a veiled attempt to get compliments.
I know I can be very critical of my own work and it sometimes takes other people to make me realize that what I did was really pretty good and I should be proud of it. I now try to make a conscience effort to not point out a chopped point or the color placement I think was completely wrong once the quilt was completed.