General:
* Scrawled
* Scribbled
* Doodled
* Scampered (implies haste and lack of care)
* Scrambled (suggests messy and disordered writing)
* Blurred
* Unreadable
* Indecipherable
* Illegible
More specific:
* Cursive (if the writing is in a hurried cursive style)
* Chicken scratch (a colloquial term for messy writing)
* Handwriting that looks like a spider walked across the page (figurative)
The best word to use will depend on the specific context and the tone you want to convey.