I smiled a lot while solving this puzzle. The theme of adding "-ing" to one word in each theme answer produced some very amusing wackiness. BUMSTEERING (68A: Hobo at the wheel?) and LUCKYSTREAKING (46A: Gangster Luciano performing a risqué prank) have somewhat obscure original sources, but they still work. My favorite theme answer is probably LIGHTSWITCHING (105A: Mild form of corporal punishment), but SQUAREROOTING (15D: Cheering done in a plaza?) is also quite good.
The fill has a lot of shortish material, but it's mostly solid. I liked seeing, for the first time ever, I think, "dunzo" written out, in the clue for GONER ("23A: One who's dunno").
25A: Dancer's leader (SANTA) was lovely. That and "86A: Drops a line" ANGLES had me fooled for quite some time. HA! And it's hard to justify now, but I kept reading 4D: How many college textbooks are bought (USED) as though it were looking for a quantity, like "a lot," and I think I needed two or three crosses before I could see it correctly.
1A: Lament after being backstabbed (ETTU) gets a B+. It's crosswordese, but it's well-clued. My first confident answer was ASSAY (10A: Test one's metal?). Cute.
So overall, the theme and the enjoyable cluing made for an enjoyable Sunday grid.