I think Jones should have taken the fight and understand why Mittrione has declined this fight. He should have kept his mouth shut though.
But the situations are completely different.
Chael is a good matchup for Jones. He should handle Sonnen without that many problems. He knows this and has said, that he can beat Chael anytime.
Cormier is a very bad matchup for Mittrione. He says this himself and thinks, that he has no or very little chance of winning. How do you go into a fight, knowing you have nothing for your opponent?
Let's say both Mittrione and Chael have a 10% Chance of winning and look at the risks of taking their fights:
- Mittrione wins 1 out of 10 times (less to lose more to win - huge opportunity)
- Jones wins 9 out of 10 times (more to lose but nothing to win - but champion)
[percentages just as an example, impossible to create realistic numbers
-> completely different situations.