While test coverage is not necessarily a direct predictor of security, any gaps in coverage can point to areas that need additional attention.
Given smart contracts are usually relatively compact yet highly significant from a security perspective, it's common to achieve 100% levels of line coverage. Using a Test Coverage Tool to assess line coverage can be a useful first check but there are many other ways of measuring test coverage.
Mutation testing is a practice where code is modified in subtle ways to see if the tests break following the modification. If the tests break that means the behavior is tightly guarded by the existing tests, if the test suite pass that means that similar underlying issues may go unnoticed.