No, it wasn’t. It was an adaptation/sequel to Dark Horse’s Tales of the Jedi comics (incidentally the best Star Wars comics ever produced by far), albeit with most of the interesting stuff stripped out.
I think TotJ is pretty goofy at times and that there are better comics, like John Ostrander's Quinlan Vos stuff. But TotJ is unparalleled when it comes to additions to the Star Wars mythos other than Lucas' movies.
For those who haven't read it, Tales introduced things like Korriban, Ziost, Onderon, Dxun, the Sith Empire, Miraluka, fleshed out the Mandalorians and introduced the concept of Mandalore etc. There's also Ossus and Jedi/Sith Holocrons, which first appeared in the related Dark Empire (written by one of the authors who wrote Tales), tying into Tales which were written afterwards.
Tales didn't only create a new setting, it created two - set apart by a millennium.
In the older era the Sith Empire and the Galactic Republic have no idea of each others' existance, until they come into contact by accident and the Empire launches a surprise attack on the larger Republic. The world really gives off an ancient vibe, with much of the galaxy not yet explored. Lightsabers have cables to belt-worn power sources and people's clothing look ancient Egyptian.
In the newer era, things have progressed with lightsabers ditching the cable and clothing looking more modern. But it's still very alien compared to the movies with the ships looking completely different, Jedi wearing varied outfits instead of unified robes and the main plot having nothing in common with the OT: A cult takes hold of one of the Republic's strongest planets, Mandalorians start raiding the Republic and certain Jedi begin to engage in less than Jedi-like behavior and search for forbidden power.
One of my favorite things in Tales of the Jedi is the character
Simus, a Sith Lord who had his head chopped off by their leader Marka Ragnos. But instead of dying, his head was put in a jar that kept him alive, and he is kept around as an esteemed advisor by Ragnos. It's just so... weird and alien, and something you can believe to have happened thousands of years before the movies.
Like the man who loves Richard Simmons said, BioWare took this setting and "moviefied" it, stripping it of the ancient aestethics and essentially re-told the story of the Original Trilogy in video game form. Not only is the change of style jarring, it's also completely nonsensical with Malak using wedge-shaped Star Destroyers (why would the Prequel-era Republic design ships in the style of space nazis that invaded them in the past?).
The only really original thing they did was the Mandalorian Wars backstory, which is not only a great story but also a logical continuation of the events in Tales.
