No, don't use rotting fresh grass clippings as mulch, they won't be very good for your plants.

If you want to use grass clippings as mulch, you need to dry the grass thoroughly first so that it doesn't form a rotting mess, which will happen within a few hours if you pile them in a heap. The best procedure is to cut the lawn without a grass collector on the mower, so they lie on the lawn spread out to dry. Then you can rake them up and they shouldn't form a rotting mess. Unfortunately, this is a lot more work than gathering them in a hopper as you mow and chucking them away. Also it isn't a good idea if your grass is very long, as it will be thick enough to make a rotting mess on your lawn.