Mental Toughness is.. the ability to handle mental pressure well-- both in holding on, and in solving the problems. Both in seizing the opportunity, and handling it gracefully.
The part about the toughness is that there are certainly different areas of toughness within person. And therefore, people have different amount of toughness. I can see how some types have more of this particular trait than others, but then again, it -may- be different in regards to the areas. Not to mention the different aspects of them...
For instance, SF generally hold their burdens pretty well, followed by NFs; but as far as taking them off, we're quite bad on that aspect.
Talking about 'fields', then we're most likely talking about mental toughness in working; dealing with deadlines, tasks, demands, and other technical problems. In this particular aspect, then yes; we are far below. I think the strongest types would be the STs, followed closely by NTs.
But there are other things.
Personal observations-- dealing with our own ugliness. To take the burdens and learn about what they talk about us. In these parts, NF wins by far; followed closely by NTs.
There are also a parts related, in regards to dealing with other people-- shrewdness, ruthlessness; accompanied by grace, elegance, wit. NTs probably top this one, followed by NFs.
I sort of ambivalent about this, but in some ways logic and rationality is also a sign of this aspect; so NTs, followed by STs, would do well.
As a quality, it shows how well they can perform under pressure. "Well" can mean many things-- graceful or quick? Offensive, or defensive? Effective, or efficient? Hits as much target as possible, or cost the least? Pleases the most people, or done the most for the community as a whole?
As a trait, it implies other things; how well they think, for instance. Probably draw some picture about their attitude and behavior (for instance, I got a vibe that they are probably more aggressive than others), but it might not be that often.
As a requirement for success; if we define success as work / career success; then yes, it is. If we define the path to success as filled with conflicts, then yes; it is.