The fact is that you should be "worthy" to being able to lift Mjölnir from his standing position.
This means that you cannot move it or lift it if you're not worthy, no matter wich tool or lever you use to try it. This is a magical attribute, so normal physics rules don't apply.
Could seem logical that, as you're not touching nor moving directly the hammer, you can tip the wheelbarrow or move it with Mjölnir inside. The fact is not, as you're not worthy, Mjölnir will refuse to get moved from the place he is, and thus, he will nail the wheelbarrow to the ground unless someone worthy tries to move it.