I do not understand your question. If I do not deviate from convention but omit unimportant details, is it a sketch for you?
If you have a project document template from PMBoK, do you feel obliged to fill all the sections or you document only information relevant to your project? With UML it’s the same: it offers a lot, but you do not have to use everything to produce a conforming diagram.
It it a sketch if you don't include every detail that UML was specifically designed to convey? Yeah? Noting that a sketch is not a derogatory phrase here.
Note that, for many of these diagrams, they existed before UML. It was an attempt to formalize them. In classes, they are often used as code generation tools in an attempt to keep a 1:1 between document and code. Laudable, but I have yet to see that work out well.
To your point, I think, the same ultimately goes for document templates. Way too many are used prescriptively as a way to make a successful project. Much to the chagrin of the document writer when the project fails. Often miserably. To the point that I'm convinced most accurate project documents are after the fact. Certainly not waterfall to the project that they are describing.
If you have a project document template from PMBoK, do you feel obliged to fill all the sections or you document only information relevant to your project? With UML it’s the same: it offers a lot, but you do not have to use everything to produce a conforming diagram.