I am not sure what do you mean by ‘the uncertainty of such parameters’?
The standard deviations of the estimated parameters are the square roots of the diagonals of the estimated variance-covariance matrix. In the nlinfit documentation, this is CovB.
The confidence interval (95% is the default) is the probability that the uncertainty of the parameter estimate includes zero. If it does (one of the confidence bounds is positive and the other negative), then that parameter is not needed in the regression model with the data you supplied to it. The confidence interval is therefore more meaningful than the variance or standard deviation.
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