well, that depends.
First, it depends on who she is facing, today's player means over 1000 players and yes, she would be able to beat most of them, now if you say top 20 it's a different story.
It also depends what Mary Jo you bring on.
the Mary Jo at her prime, teletransported to the current times?
Oh Mary Jo, like she was, physically and technically, but evolving to meet today's requirements.
In the first case the answer is simple, any top 20 player (and many top 50) would simply trounce her. Yes, she did challenge Serena that later won the US Open but Serena was inexperienced then.
Now, if Mary Jo, with the skills she had, played in today's environment, in other words, she developed her tennis in today's environment, then she would have a reasonable chance.
She wouldn't beat Serena, or Justine, or Kim, or Lindsay, or Sharapova (at least on grass) if they are playing well, but she could be dangerous if they are a bit off, same applies to other top players with the difference that I think she could beat for example Myskina, even if Myskina is playing well.
She would consistenly beat players outside the top 20 but she would have trouble handling today's pace, especially against someone like Serena, or Sprem, she would probably do better against Kim or Justine because her game would suit them better but she wouldn't win if they are on.