I’m pretty sure critical hits (5 or more Skulls) cap out at +2.
(Doesn’t specify what happens with more than 5, but:)
(Ignore what Kafka said afterwards)
If it’s bugged, I don’t think it’s been fixed.
I would put it at 54 (base attack) +2 (crit) + 7 * 5 (Doomskulls) = 91, and then I guess +5 from the secondarily-exploded Doomskull at the top (as Sytro mentioned).