Believing that too many people go to jail for minor offenses or those which don’t endanger others isn’t incompatible with believing major crimes should still have jail time.
What’s especially worth considering here is what Holmes did: most people expect high standards for medical ethics since lying as she did could have irreversible impacts on people’s lives. Her fraud involved inaccurately telling people that they had cancer, or giving values which lead to changes in medication or other treatments. That isn’t as bad as it could have been – I don’t believe anyone was given a false negative on cancer, for example - but it’s absolutely toxic for trust in the medical system if it’s not strictly punished.
Similarly, as a corporate officer she lied repeatedly to investors, regulators, and the public.
What both of those have in common is that they’re positions of trust where fraud is relatively hard to catch. Millions of people are in positions where they could on a daily basis abuse their customers’ trust and potentially never be caught, and so we have to consider the deterrent effect as well. Doctors, bankers, CEOs, etc. are highly paid so a financial deterrent isn’t nearly as effective at preventing lapses as the possibility of jail time - your family trust can’t serve it for you - and those consequences also mean that the people around you aren’t going to help cover it up the way they might if it was profitable enough to look the other way.