I had made many meads by the time I took and passed the Mead exam, including my share of crappy, off-flavor meads, before I really took temperature control seriously.
If I remember correctly, the online exam was heavily weighted towards being very familiar with the Mead Style Guidelines. Being very familiar with the style guidelines will also be key to passing the mead judging exam. In every section of the scoresheet, in addition to describing your perceptions of the mead you are judging, it's important to comment on how those perceptions do, or do not, match up with the style of mead you are told to judge it against.
So if you do nothing else, study the style guidelines, over and over.