Mummy isn't really risky at all; I'm not counting opportunity cost in relation to Weak Armor, since every ability supplies that. The chance of hurting yourself with Mummy is so much lower than the chance of hurting an enemy with Mummy that I can't really see this being a "high risk high reward" ability. Hitting another pokemon with mummy means you ate a physical attack with CAP4, then switched, then successfully use a physical attack on the enemy. That requires three turns to go a particular way, as opposed to one for just infecting an enemy.
That said, I don't think it's a horrible choice as a secondary ability; it's the best choice on the slate if we want the secondary ability to boil down to a choice between Weak Armor or no Weak Armor. It may sound like I just dismissed this argument in my first sentence of this post, but I am really making a distinction. There are a plethora of abilities that could accomplish the same thing (namely, not Weak Armor and not that influential), and they aren't risky abilities; choosing such an ability may be fine for the concept, however. First, it works as a great "control" for Weak Armor, as being able to play CAP4 with either ability will let us decide whether or not the Speed boosts are worth the Defense drops. Second, it is another ability based on taking physical hits, which will make life a lot easier when we reach the stats stage.
P.S. The number of people suggesting Hustle is kind of alarming when it was pretty clearly agreed upon (and stated by BMB more than once) that we are NOT using luck based risk on this pokemon. I can only guess it comes from people who didn't notice this CAP until after the early stages (where this was decided) and don't know this. I think it would be awesome if there was a list of links to the TL's post, which could be linked in the "CAP so far" part at the beginning of every thread; this might make it easier to catch up important decisions that have been made about the CAP that haven't been a result of a vote. At the very least, his posts in the Concept Assessment and Threat Discussion pages could be linked to, since those decisions affect every subsequent stage.