Maybe among hardcore jazz fans... but if you look here it seems many listeners consider it to be his "best" album.
Probably because it has quite clearly defined 'tunes' (or attractive chord progressions) that make it easy for a general listener to get a grip on. I still wonder how many of the germs were plucked out of thin air; how far he had them stored up to use.
It appeared at just the right time. I suspect many young listeners, like myself at the time (about 20), were wearying of rock and its hard and fast beat; yet the likes of Bill Evans still sounded 'old-fashioned' (a perception at the time, not a judgement on the music) and the likes of Chopin and Rachmaninov too fusty. Jarrett offered something that seemed free-wheeling, unencumbered with a regular beat yet with the melodic interest that most listeners need. I came to his solo music via Facing You and Bremen/Lausanne so Koln was not the surprise that many heard.
Listening back I can appreciate the criticisms - if you'd been listening to Paul Bley, Mal Waldron or Andrew Hill than Jarrett would definitely sound over-perfumed.
I can still enjoy Jarrett - including Koln - as part of a broad, balanced diet because that was my entry point.