I recently read about an outrageous court decision in my country where the prosecution only asked for a 2500€ fine for a dad for torturing his kids for years. The defendant obviously agreed to it (its not a particularly high fine in my country) and the judge was OK if everyone is OK with it so it was a shortened trial.
I don't know the details of the case, but you would have to distinguish between legal errors or disputable points of law (including those which might have a bearing upon you), and disagreements with political or judicial policy.
It sounds like your interest is the latter. There are very limited rights of court appeal against political policies decided by elected politicians, and there is no right of appeal against judicial policies correctly applied by the judge (and the judiciary control whether they think there is a possibility of error in applying policy, and whether they want to hear an appeal).
Generally speaking, the criminal justice system is less vindictive than a significant proportion of the public would wish it to be.
This is so for various reasons, including a desire to be economical with resources, and studied opinion on criminal justice matters and offender management being different from those of the public who are engaged with neither the details of problems nor the historical experiences.
Unless some kind of dishonest wrongdoing is alleged on the part of officials, then challenging a settled arrangement which was offered by a public prosecutor and accepted by a judge is unlikely to lead to any alteration in the penalty.
Often, a lenient penalty offered for what seems to be a sensationally evil crime, might reflect the weakness of the prosecution case which would only be apparent to someone with full access to the evidence and experience with its interpretation.
Another possibility, sometimes, is that the perpetrator may be a "licensed" criminal, and the weak sentence reflects a desire to keep them out on the street informing or acting against other criminals. This information is submitted to the judge privately and is not made public.
All told, there is very little power for an individual to directly challenge the outcome of a case in which they were not a party, on the mere grounds that they do not agree with a sentence or a plea deal.