Perhaps the unskilled laborers' efforts may be best channeled into loosening the earth so as to leave the druids, sorcerers, and wizards to focus on assembling it into the ramp outside the clerical stronghold.
Well, if they were disturbing compacted soil, that would make sense. It's not like a Motte couldn't be built of compacted soil, it just takes times & effort. Add 8" of soil, pound it until it is only 6" thick. You could run a herd of sheep across the freshly deposited dirt for 1-2 hours (sheep have an extremely high weight/foot area ratio) or have people walk around with a heavy log they thump on the ground (aka a tamper).
Plus sides of a compacted soil motte: it is resistant to Mold Earth and requires more effort from sappers.
Plus side of a loose soil motte: it is more likely to collapse on any sappers or anyone doing excavation immediately, forces them to do more effort, and your Mold Earther can collapse the pit on them. Restrained targets are easy targets.
General rule of thumb for loose/uncompacted soil is use a 3:1 cut back. Go down 5ft, pull the top back 15ft. Otherwise you need shoring. I.e. to dig a safe 10ft deep hole, it needs to be a pit 70ft across. Loose sand will collapse immediately and you have to do that amount of excavation.
Compacted soil will eventually collapse as it loosens along the disturbed edge but "eventually" might be in a month or a year.
Table on angle of repose for loose vs compacted soils:
structx.com