The whole idea to me is that SSJ for Gohan was completely unnecessary since he was drawing out all potential power in his Mystic form.
The original translation goes something like this:
"How do I become the strongest fighter?"
"Oh, you just do what you do when become that Super-whatsit."
"Like a Super Saiyan, huh?"
HAAAAAA
And then he's Mystic Gohan. Basically it's the same as in those bootleg manga panels. The key is that the Elder Kaioshin describes the process as like becoming Super Saiyan--meaning this could mean that Gohan simply has to "power up".
The debate lies in whether or not this is considered a transformation, or if he was just "unlocking" all of the power that Elder Kaioshin brought to the surface for him.
I've always been under the idea that Gohan COULD go SSJ, but it would literally be a waste of Ki, just like Vegeta, Trunks and Goku going Super Saiyan Advanced 1 and 2 is a waste of Ki.
I think this theory is corroborated by BOG. Gohan went SSJ in order to turn Goku into a God, but when fighting Beerus he simply powered up in his normal state.
SSJ is an obsolete transformation to Gohan in the same way that Toriyama describes SSJ2 and SSJ3 as obsolete for Goku and Vegeta.
Or at least it was. Now this movie comes out and Gohan is turning SSJ to fight Frieza's soldiers
.
I'm going to agree with Jays and assume that Gohan simply got weaker, and his base form likely does not have the potency it did against Buu. That doesn't mean he's no longer "Mystic", just that he got soft the same way he did after Cell.
P.S.
THE WIKI IS TRASH. NEVER REFERENCE IT IN A DEBATE