My view on this is that ERPNext interface is simple, clean and flexible. But it is not intuitive.
In many cases, looking at a menu head will not giving a hint of what is inside.
So, as @rmehta suggested, a training is inevitable for the user to know what’s inside before using it. Or, also as he suggested, for the user to use it for 6 months and then he knows what to change/customize.
Many suggestions to use the search field to find a form or functions shows this case. It should be “searching” not “finding” 
I kind of agree with @barhamed, but rather than to simplify the interface, make it more intuitive. And it doesn’t need to be a simplification (let alone reduce) of the features as mentioned by @tmatteson.
My clients are SMEs. I don’t “sell” this software to them as ERP. I told them this is for supporting their operation (good thing for using ERPNext is that we can use it partially). So they can use it part by part as they grow familiarity with it.
My method: Recognize the function (from the real operation) → Record static database → Record transactions → Apply the business process → Do report and analytics.
Anyway, I still rearrange the menu (sidebar and pages) to be more intuitive for them.
As mentioned: “ERPNext is a fully featured ERP system designed for Small and Medium Sized business” so migrating from pen-and-paper is common.
Regards