Now it's a strong claim to say no here but there doesn't seem to be any evidence that she is a Super and there is a case of someone similar who was not one.
The Supers were born with their powers
In my answer to the question: Were the Supers born or created? I come to the conclusion that the Supers were born with their powers.
Syndrome is perhaps the closest person we've seen to Edna in terms of his intelligence and what he can create with technology. Yet he is not a Super, or wasn't born one anyway (implying from the above that he wasn't one).
Syndrome wasn’t born with superpowers, but envied the attention and admiration the Supers received. Of all the Supers, Mr. Incredible embodied everything Syndrome desired: strength, fame, and popularity. Highly intelligent and cunning, Syndrome now spends his days on a remote island on the other side of the world inventing gadgets and robots to carry out a diabolical plot.
Pixar, The Incredibles, Syndrome
Now onto Edna herself, on the Pixar pages for both films she is described as working with the latest technology and being able to use it very well. No where is it mentioned that she is a Super or has Super intelligence, just that she's certainly at the top of her field.
Brilliant and successful, Edna Mode got her start in the industry as the world’s leading superhero costume designer. With her now-mature sense of design, she remains the top designer in the field, taking her clothing designs to Milan, Paris and other internationally important fashion centers. However, E longs for the return of the superheroes, for a real design challenge, so she can fuse the latest technology with her impeccable fashion sense and unfurl her incredible creations for the adoring public.
Pixar, The Incredibles, Edna "E" Mode
Edna "E" Mod possesses impeccable design sense, a keen understanding of cutting-edge technology and an unmatched skillset. A creative visionary, she longs for the return of Supers so she can once again create functional yet edgy supersuits. The petite and powerful fashionista treats her clients like family, dahling, but sticks firmly to her long-held assertion: No capes!
Pixar, Incredibles 2, Edna "E" Mode
With a lack of evidence to say she is a Super and evidence that is probably there to say she isn't we have to lean towards no.
To answer the actual "theory" that she is a Super...
Edna Mode may have had some sort of precognitive powers, since she seemed to know Jack-Jack would have powers before anyone else seemed to.
This doesn't really seem to be the case, as far as I can tell she just assumed Jack-Jack had powers because Bob and Helen do and both of their first kids, Violet and Dashiell, did.
Edna: Well, I'm sure I don't know, darling. Luck favors the
prepared. I didn't know the baby's powers, so I covered the basics.
Helen: Jack-Jack doesn't have any powers.
Edna: No? Well, he'll look fabulous anyway.
It also posited that the seeming government connections she has might imply their choice to have a Super design for Supers.
Whilst I have no evidence either way here the following quote (taken from one of the quotes above) does say she wasn't the only designer for the Supers. This doesn't exactly say she isn't a Super but does poke a hole in the theory.
Brilliant and successful, Edna Mode got her start in the industry as the world’s leading superhero costume designer.
It's been pointed out that this quote is ambiguous and could read as (superhero costume) designer or superhero (costume designer). I personally believe that it is the former from the context of the quote but if you want to interpret it as the latter that seems valid too.