"smoother" would generally be preferred, but "more smooth" can be used too, and sounds ok. The water was choppy. Is the water smooth in the morning? Yeah, it's smooth. (one possible answer) It's more smooth. (i.e. not smooth, but smoother than the comparison. It's easier to say "more" here than trying to emphasize the "er" if you say "smoother")

by itself "smoother" is correct. and more smooth is controversial to many speakers and writers some say YES and some say NO. A lot would depend on which sentence clause it was in and how it was written. You can definitely say "more smoothly" if describing something running moving operating functioning more smoothly after adjustments alterations improvements maintenance and a overhaul and a little oiling or greasing etc. I believe "more smooth" is wrong because "smoother" already contains the "er" that "more" is adding to smooth. More is typically and usually added to a quantity or an amount. Not an adjective so those who argue it is OK to say "more smooth" are not correct, a smooth surface can not be "more smooth" it can be only "less smooth" "more smooth" would make it "flatter" <--flatter contains the "er" that "smoother" contains. You have two choices two words to describe a surface that has been polished sanded planed etc "smoother" or "flatter"<------ Excluding any scientific or industrial or fancy writing words that may describe the same thing.

Smoother - (monosyllable)