Well, as usual, Marvel wants us to speculate
And then they will retcon whichever they see fit in the next movie (like they did with Loki's scepter). personally, I feel following are possible :
1. Any infinity stone over rides Odin's enchantment. As such in the comics, the stones/gems are the most powerful artifact and no one even knows their full power/capability.
2. His intension was pure and he wanted to save humanity or "life" as he says. So he's deem worthy, at least in the duration of the movie (Yes, many characters have been able to lift and wield the Mjonir when the situation demanded it and thier intension was pure and selfless making them worthy for the moment).
3. He's a machine and hence doesn't fall under the category needing to be deemed worthy. They never showed him wielding the magical power - he just used it to hit ultron once as a normal hammer, didn't conjure thunder nor he used it for flight since he already had the capability. So yes, similar to what Stark said a lift being worthy.
4. He was born just a few moments back and hence like a bay, the Mjonir couldn't determine his worthyness.