Both mining nodes and non-mining nodes are important for any changes to the Bitcoin network protocols or to the consensus rules.
The Bitcoin network protocol itself doen’t distinguish between miners and non-miners. Initially all nodes were mining nodes.
There are mechanisms by which miners can signal readiness for a proposed change by setting a bit in the header of blocks they create. Obviously this mechanism isn’t available to non-mining nodes since they don’t create blocks.
However, miners cannot force non-mining nodes to update or change their software. There is no mechanism by which they unilaterally impose changes, other than market sentiment etc. So, in most cases, it probably isn’t appropriate or useful to place either group on some scale of relative importance.
So far as I know, changes to the Bitcoin network protocols are mostly (exclusively?) extensions that add new message types etc without removing old ones that are used by older software. This makes it easy for old software to continue to interoperate with new software and makes it relatively painless to update software to accomodate the new features.
