Can't agree with that, she's not had good luck with blockbuster roles with the prequels being terrible and her role in the first two Thor's being pretty bland but that doesn't equate to being a bad actress, did anyone but Ian MacDiarmid look good in the prequels?
Plenty of good roles elsewhere in her career and plenty to suggest she could be fine in a well written blockbuster, she's maybe been typecast a bit of "woman having an emotional breakdown" with stuff like Black Swan and Jackie but can play up more personality when needed, Closer for example.
Johansson's personal trainer won't be out of work anyway even if Widow stays dead.