Can ≈ (approximate equality) be used for...

Is it a formally acceptable use of ≈ to express arbitrarily decimal rounded numbers?
For instance, 4/7 ≈ 0.57.

Also, is 'decimal rounded' an accepted term?

Yes. That symbol is perfectly acceptable in that case. Especially if the fraction is irrational in decimal form, in which case you would HAVE to use that symbol to be mathematically correct. You may want to indicate which place value you have rounded to, or at least stay consisting in your rounding. i.e. always round to the thousandths place, etc. I personally have never heard "decimal rounded." I am a senior mechanical engineer at Clemson, and professors always just say rounded, or rounded to the whatever place. Never "decimal rounded."

Yes, 4/7 ≈ 0.57
0.57 is 4/7 rounded to the nearest hundredth