What is the weather like in Interlaken Switzerland in mid-June?

Will I need a ski-jacket (layered jacket) kind of thing or will it be warmer than that. I do feel the cold. Thanks

Warm for most of the time. Summer in the Bernese Oberland can be quite wet sometimes so take a folding umbrella.

Defiantely do Not want a ski jacket. You'll get a gentle breeze from the lakes so an ordinary fleece coat will be quite enough.

If you go deep into the Bernese Oberland (e.g. to Grindelwald, Kleine Sheidegg, etc) via the BOB train then you may want to take an additional thin shirt.
If you're going to walk the mountain peaks then you'll need the extra thin layer because it can be nippy if a low cloud engulfs you.

A trip up to the Jungfrau can be cold in the shade because it's permanently snowy. If you stay in the sun when up there you will get warm so don't overdress. Have gloves.
Sunglasses are a must because of the bright snowfield. (hint - don't drink the Swiss beer up there, it's lousy stuff).

It is important that you take stout shoes or hiking boots if you intend to do any mountain walking (which you must).
There are good paths for much of the way but there's also a lot a gravel and that's tiring if you're just in trainers. The local insects also like to eat tourists ankles.

Be comfy in the hills, take some light walking boots. It's also nice when you change into your shoes afterwards before going into the bar.
Wear the boots for flying so that they don't raise your luggage weight.

Note that most of the cable cars close by about 6pm and the mountain railways don't run very late either; so you must be careful not to get stuck at the wrong end of one.

No, summer clothing is sufficient unless you want to go up the Jungfrau or similar.

The weather in Europe has been greatly affected by global warming. I would guestimate that you will be fine with a light jacket. You can probably expect the temperature to be in the 60's or 70's.