Is it o.k. to cut off the dieing flowers of the rodadendrum?

Use your fingers, bend and snap the spent flower part at the very bottom (above the new leaves), being careful of the new foliage.
And it can be a sticky process.

If there is anything about the shape of your young rhododendron you might want to improve, say if you have a longer stem that makes it look unbalanced to you, you can easily improve that now too by intentionally snapping off new foliage buds. Doing that now does not affect next year's blooming.