Condensation or water vapor - nada problemo - absolutely normal

It might mean that ice is developing on the evaporator and forcing air
around the sides.  Normally the evaporator fins collect the fog out of
the air.  Check the air temperature with a thermometer to see at what
point your AC cycles on and off.  It shouldn't reach freezing
temperature.  The thermostat might be bad or it may have fallen off the
evaporator.

As for what your mechanic said, Honda ACs are typically underpowered and
the condenser is placed under the bumper in the path of flying stones.

This is a very common concern with most civics and alot of other brands.
this is a normal characteristic.if you switch to fresh air when this
happens the misting will subside. it is the same effect as opening your
freezer door on a hot humid day. the honda civic does have a very small ac
condensor there for making it work very hard on those sticky days.

I have worked as service manager for honda for 11 yrs. I cant begin to
tell you how many times i have had this conversation.
Drive your civic have fun. No worries.