I plant clover with a first-year annual cover crop like wheat, sunflower or some such thing. Wet or dry, I don't mow the first season, but then hay it the next year. It doesn't do it any harm.
The answer is yes, mowing the first year will improve it like the other guys said. However, leaving it alone until the first haying next year is also just fine. It certainly won't hurt it.
BTW: One of the nice things about clover that I've found is that it seems to thrive on being mowed. Other stuff, dwindles when it's mowed. On the right clay soil clover kicks into high gear. An example is my own front lawn at home. I had a water main break two years ago, and I put the local hardware store's mix down after they dug up my yard. There was some Ladino clover in it. Now that part of the lawn is almost 100% Ladino-- even the fescue is crowded out.