If you have a well-defined standard against which you can measure, it's a good method, if you have none yet, it isn't.

If you don't have any standard to compare with, the most accurate method is probably evaporating the solvent carefully and weighing what's left. If you have a mixture of species, you can also perform preparative LC and the continue with evaporating-weighing.

A more standard approach is to use a spectrophotometer or nanodrop, those will give you ug/ul and you can just convert the units.

Or you can run out an aliquot on an agarose gel and compare the brightness to known concentration of a DNA ladder. This will give you a "reasonable enough" approximation for things like cloning.

I've never heard of a molecular biologist using LC-MS to quantify a plasmid.