In my eyes it is fairly good
Lax nature lowers Special Defense but increases Defense. It is good to me because most of the Ground-type moves are Physical with a few exceptions. The strong Ground-type moves, like Earthquake, are Physical so with a +Def nature it can tank hits from them fairly well. A +Def nature also works because it has base 79 def.
As per the ability it is awful.
Rivalry is never recommended on the Luxray line or on any Pokémon in general so you should definitely change it and get Intimidate. If you are lucky enough, you can also take HA Luxray, which has Guts and is pretty strong.
So to summarise, Lax nature is good and Rivalry is awful.
This is the source of the natures list I referred to.