The large spectral libraries can be useful to gain a first approximation of the relative fragment ion abundances you should expect to see in SRM/MRM, especially for y-ions. There are several papers about this, but here is one from our group:
However, since you frequently don't know the instrument on which these library spectra were measured, they will not be as accurate in the long-run as measurements on your own instrument.
Here is a paper with some insights on generating your own spectral libraries, but on a scale large enough to support DIA/SWATH, which is likely not necessary for SRM/MRM. If you have synthetic peptides, researchers will often start by measuring the synthetic peptides of interest to get high-quality MS/MS library spectra for the peptides that will be targeted.
Using public libraries is generally seen as a good starting point when you need something, but not a complete substitute for your own empirical measurements.
Some groups have taken the concept much further, though, using further experimentation to develop robust assays, fully characterized over several experiments for linear range, repeatability, etc. such as the CPTAC Assay Portal:
In summary, you should give some thought to the source of your library spectra and how closely you expect it to match your experimental conditions and how you can get out of the library measurements in prior knowledge for your own experiments.
Hope this helps. Sorry the answer leaves so much to interpretation, but that is the state of the field.