Does a 3/8 valve instead of a 1/2 valve on a gas stove matter?
As long as the oven only needs the output from 3/8" then there is no issue with this at all.  I would think that that 3/8" could supply any home oven (that's still a lot of gas) so I don't think there is an issue at all.  If the manufacturer agrees then there is no reason for you to worry about it.

Frankly the installer probably installed the shut-off that he normally installs for ovens.  Most ovens are fed with 3/8" or 1/4" so he might not even have had this on him at the time.  

This is akin to having a 1" water line going to a bathroom shower and a plumber using a 3/4" shutoff for it.  There are almost no shower valves that can even take 3/4" of water and certainly not 1".  Lazy I don't know.   If your shower has inputs of 1/2" or less it doesn't matter.  Just like if your oven can only take in the output of X (which may be less than a 1/4" feed) then it doesn't matter.  It only matters if he installed it right.