In its immediate context, the comment refers merely to physical clothing. I hadn't considered before that what God does for Adam and Eve at the end of Genesis 3 (verse 21: "And the LORD God made for Adam and for his wife garments of skins and clothed them") is a specific example of this, probably the epitome. The phrase "'much more clothe you'" reminded me of 2 Corinthians 5:4, though, which seems to refer to clothing in a different way: "For while we are still in this tent, we groan, being burdened - not that we would be unclothed, but that we would be further clothed, so that what is mortal may be swallowed up by life." In light of that context, I think the clothing in Matthew 6:30 and Luke 12:28 can also be viewed more metaphorically, as it is in Isaiah 61:10: "He has clothed me with the garments of salvation; he has covered me with the robe of righteousness." God clothes us physically, as He does the lilies of the field, but He also clothes us metaphorically by giving us that salvation and righteousness.