In general does the phrase “[adverb] average” follow the rules of proper English? For example I asked of the water level of a forest and someone replied that it was “very average for this time of year”.
There is nothing wrong with the syntax adverb adjective. It is true that the expression itself does sound a bit strange but I would not consider it incorrect since the adjective average can be interpreted as “usual or typical”. (Not necessary to be interpreted as “constituting a numerical average” all the time.)
[a search result from COCA for “very average”]