Assuming they did not enable "Find my iPhone" or whatever it's called on iPads, yes. If it is enabled, then unless they do a remote wipe, it's almost permalocked be it in regular or Lost mode and it will randomly ping back to the owner's iCloud account via Apple Maps showing exactly where it is.
If it has a pin code pass it's asking for, I'm sure there's a way around that by booting into recovery. But, as I mentioned, if Find my iPhone is enabled, it may as well be a brick unless the owner remote wipes it.