Q: Jungfrau top of Europe visibility ?

wait until you get there.

When it is good it is very very good. When it is bad it is awful.

Impossible to predict. Look at the daily forecast, and make a final decision the night before.

September (and the rest of the Autumn) is famed for "Blau über weiss" (blue over white), the valleys, for example down in Interlaken are filled with cloud, but once you get high enough it is clear. If you are lucky enough to get one of these days you will have superb views.
Click here for a photo of this phenomenon.

Mountain weather is the kind of weather that can change in a matter of hours, let alone over days or months. You can find averages but there's no way anyone can predict what the visibility will be like in mid-September at this point.

I know exactly what the weather will be in that region in mid-September. However, I am keeping that information secret until late September.

Chris F., we had that exact situation when we went up to Mt. Rigi this past November: cloudy on the ground, but as the cogwheel train broke through the clouds, a collective gasp as the incredible blue skies reigned, with mountains peaking through the cloud layers. It was as magnificat a sight as I've ever seen, with fresh snow seeming to meet the clouds.
Safe travels!

There is a webcam at Jungfrau. Many train stations and even hotels in the area have monitors tuned to it so you can see conditions at the top. If it looks good, go. If not, wait for another day (if you can).
https://www.jungfrau.ch/en-gb/live/webcams/#webcam-jungfraujoch-top-of-europe

I'd suggest checking both the weather reports for the day you plan to visit the Jungfraujoch, and also check the webcams before you go. If you have to make the trip on a specific day due to your Itinerary and the weather is less than favourable, you'll still be able to tour the ice displays and other indoor sights. If you want to venture out on the platform at the Sphinx Observatory, be sure you have a warm coat as it can be cold in September.

One final point to mention.... The Jungfraujoch is at an elevation of about 12,000 feet+, and that altitude may affect some people. At the least you'll probably find that it takes more effort to climb stairs and things like that.

Weather forecast there is very unreliable. Take your chances and you will be rewarded - like we were (last September) or not. It was raining several days, no visibility. We went up by cable cars around Lauterbrunnen valley and it was awful. At the end of our trip we took the train up Jungfrau. Some people from our party already gave up and went to caffe. We felt almost like most people feel when they buy lotto. They know they are not going to win, but what if? Well, we won. All that fog and rain stayed down. We were welcomed by sunshine and all these shiny peaks. When we showed them our pictures from there our friends were very sad that they gave up. I would say, take your chances and go.