Why are isotopes an issue in reading mass spectra?
Here is the "periodic table for biomolecules" (leaving out hydrogen, please ignore silicon):



The pie chart shows the isotope distribution. Of the elements C, N O, P, S and H, sulfur has the highest abundance minor isotope (around 4%). On the other end of the spectrum,phosphorous is a mono-isotopic element, with just one stable isotope. There is an interactive version at https://www.isotopesmatter.com/applets/IPTEI/IPTEI.html.

So for small metabolites, there will be little signal due to minor isotopes. However, in a macromolecule with hundred carbons, the 13-carbon isotope (around 1% abundance) does become relevant. The signal for the mass ion will approximately follow a Poisson distribution, with zero or one 13-carbon isotope in the molecule as most probable:



Source: https://homepage.divms.uiowa.edu/~mbognar/applets/pois.html

For smaller numbers of atoms in a molecule, the multinomial distribution is appropriate, as explained here: https://chemistry.stackexchange.com/a/87669/72973