I suspect the truth was she did not like capes and considered them dangerously unfashionable. From her perspective, she thought capes ruined the line of what she considered the ultimate expression of superhuman appearance, the well-garbed human form.
Rumor has it she was likely responsible for issuing said capes to the supers (against her fashion advice, of course, darling) and may have had some indirect involvement in their demise.
Capes added more lines, a madly fluttering distraction and had potentially lethal consequences to their daily use. Having memorized half a dozen heroes who met an untimely end due to their use of the deadly fashion accessory, she felt fully justified when she emphatically stated: "No Capes!"
Bob is trying to get Edna Mode to repair his mega-mesh super-suit damaged in combat with Syndrome's weapon.
Edna: You need a new suit, that much is certain.
Bob: A new suit? Well, where the heck am I gonna get a new suit?
Edna: You can't! It's impossible! I'm far too busy, so ask me now before I can become sane.
Bob: Wait? You want to make me a suit?
Edna: You push too hard, darling! But I accept!
Edna: It will be bold! Dramatic!
Bob: Yeah. Something classic, like, like Dynaguy. Oh, he had a great look! Oh, the cape and the boots...
Edna: [throws a wadded ball of paper at Bob's head] No capes!
Bob: Isn't that my decision?
Edna: Do you remember Thunderhead? Tall, storm powers? Nice man, good with kids.
Bob: Listen, E...
Edna: November 15th of '58! All was well, another day saved, when... his cape snagged on a missile fin!
Bob: Thunderhead was not the brightest bulb...
Edna: Stratogale! April 23rd, '57! Cape caught in a jet turbine!
Bob: E, you can't generalize about these things...
Edna: Metaman, express elevator! Dynaguy, snagged on takeoff! Splashdown, sucked into a vortex!
[shouts] Edna: No capes!