To emphasize the act of managing poorly:
* Mal- (meaning bad or evil): Malmanage
* Mis- (meaning badly or wrongly): This is already present in the word, but you could emphasize it by saying mis-mismanage for a humorous or ironic effect.
To imply a specific type of mismanagement:
* Self-: Self-mismanage (to mismanage one's own affairs)
* Over-: Overmanage (to manage excessively)
* Under-: Undermanage (to manage insufficiently)
* Macro-: Macro-mismanage (to mismanage on a large scale)
* Micro-: Micro-mismanage (to mismanage on a small scale)
To imply a consequence of mismanagement:
* Dis-: Dismanage (to manage poorly, resulting in disorder)
The best prefix to use depends on the specific context and the meaning you wish to convey.