Lawn clippings only mulch
I have been collecting lawn clippings for about 10 years, all in one large pile - the new clippings go on top every time.
I haven't dug into the middle of the pile, but I would assume that the clippings would be well broken down.
As there is nothing in the pile but lawn clippings (apart from some leaves that would be in with the clippings in October) would these be OK to use as mulch. I'm not certain whether near 100% lawn clippings would be good for a border / planting area.
If not I agree it should be fine to use. I would only use the well rotted stuff. I've used fresh clippings as a mulch and ended up with lots of grass growing in the borders.