I would say your interpretation is not correct. Just theoretically tells us pizza does not merely taste about as good as pasta: it tastes exactly as good. So it cannot taste slightly less good: it truly is up to the level of goodness of pasta and not an iota lower.
In practice, this mainly serves as a mild intensifier to the expression as a whole; it mostly just adds emphasis.
Just doesn't suggest that pizza is better than pasta any more than the variant without just does (they may both equally suggest that it is better, or not, depending on context).