The definition of "mobile games" come from being released on a mobile platform first. It is certain that the game is played more on mobile devices, whilst its 1st release came from a mobile device.
Take Minecraft. Is it a mobile game? No. It first came out as a PC release as what we know as Java Edition. Soon then, Mojang decided to create a Minecraft which is playable on mobile. This version would be called Pocket Edition, written in C++, meaning its incapable of playing with PC servers. Soon, after the realization that all controller, mobile, and Windows 10 versions are written in the same language, they combined it to become Bedrock.
So is Bedrock Edition still considered a mobile game? Depends. It does have a big part of the users playing on mobile, still.
Whatever the case, a "mobile game" is basically a game originating from a mobile platform.