“Mother lode” in today's Tundra is used metaphorically to decribe something of great value. Velma's going to make one heckuva nest after scoring an unfortunate man's toupee. (In my humble opinion, cartoonist Chad Carpenter could've had a little extra fun if he had drawn Velma as a bald eagle.)
In its literal sense, "mother lode" is an Americanism that means “the main lode, or vein of ore, in a particular region or district,” according to Webster's.
“The term probably came from a literal translation of the Spanish veta madre, a term common in old Mexican mining. Veta madre, for instance, is the name given to an 11-kilometre-long (6.8 mi) silver vein discovered in 1548 in Guanajuato, New Spain (modern-day Mexico),” Wikipedia notes.
Carpenter relies on the reader's understanding of what a toupee is in order to get the joke. Coincidentally, another cartoonist — George Lichty, creator of Grin and Bear It — also had fun today with the idea of a wind-ruffled toupee: