Pinkie Pie takes great pride in throwing a welcome party for every new pony (or person) who comes to Ponyville as the official welcoming committee (self-elected, of course). As a baker, she takes great pride in using the best ingredients in the treats that she makes; no synthetics, no substitutes including sugar.
 
Mac, as an 8-year old kid, is well-known for being reasonable, creative kid. He is also well-known for his complete and total inability to handle sugar in any capacity. He turns into a rather rabid-like creature who seeks out sugar and other sweets to fuel his relentless rampage.
 
Somebody REALLY should’ve told Pinkie that before she made him eat those cupcakes.