Use 'muck about with' when referring to casual, non-serious activities.

Playful Context

Implies time could be better spent; often used to call out unproductive behavior.

Time Wasting

Can be used to mildly scold someone, especially in an affectionate way.

Describes informal, unresearched changes made to something, often technology.

Warns that changes made without knowledge can lead to problems or damage.

Suggests that the person is not an expert in what they are trying to do.