Will he fail as hard at Aquaman as he did as Conan?

There were a number of things wrong with "Conan". Jason was not one of them.

Agreed. He was awesome in Game of Thrones.

You have a virtually unknown set to replace Arnold, that's a pretty big issue.

Arnold was virtually unknown when he filmed Conan as well, but his legacy is a difficult one to replace.

The two Conans were quite different to my mind.
Momoa's Conan was meant to be more like the original character from the Howard stories. Arnold's bore significantly less of a resemblance, partly because Arnold could barely speak English.

Arnold was famous for bodybuilding back then

Yep! Arnold's nickname was the "Austrian Oak", and played the Conan character like a big tree stump. As for whether Momoa will fail as Aquaman? Well...he's
not following in anyone's footsteps this time. He's not an unknown actor anymore.
If his Aquaman character does make a cameo in the upcoming Batman vs Superman
film (which will be seen by millions), it would make a great lead-in to the Aquaman movie. However, if Aquaman's appearance is altered drastically as to
make him almost unrecognizable to his fanbase; there could be major blowback.
Lose the long hair and beard (no King Neptune). Put some blond streaks...and
viola!

He has no talent at all.