A nucleophile should get better at its job if it can donate a pair of electrons more easily. We say that a more electronegative atom should be less nucleophilic , generally. I understand this is because the more EN an atom gets, the more strongly it pulls electrons towards itself. But why does the fact that such an atom would ,generally, have more electron density not override this?
Electronegativity and nucleophilicity are different concepts. Electronegativity refers to how strongly an atom pulls bonding electrons towards itself. Nucleophilicity refers to ability of atom to donate a lone pair of electrons. Different factors affect nucleophilicity, such as orbital size, charge of ion, size of molecule, hybridisation etc etc.