I just want to set a record straight. I think Rest, an OHKO, is the Best reason to play Jiggs. I do NOT think it's the Only reason to play Jiggs.
A character that Jiggs cannot instantly kill is a bad matchup. What type of barrier would cause the opposing character to be safe and still attack Jigglypuff? The only type of hitbox that could do that is a disjointed hitbox. What characters have disjointed hitboxes? Marth and Link are good examples of characters known for their hitboxes being disjointed.
How would one go about countering these characters? If Marth or Link can get hit either in the start-up lag or ending lag of their attacks (which is short, but possible), then their attacks will be negated. What characters counter Marth and Link? A good example of a fast character is Fox, or MetaKnight, who have little-no startup or ending lag (Fox's SHFFL'd N-air is a great example; it comes out almost immediately and Fox has great horizontal movement on his short-hops). So if it's match 2 and someone tries to counter my Jiggs with a Marth, I counter their Marth with Fox.
Just don't get grabbed by Marth while playing Fox.