If you divert from overhead destination with your normal reserves, you should be able to fly to the alternate and land still with 'final reserve fuel' on board; there is no need to declare an emergency of any sort. If you are go below final reserve fuel, you must declare a Mayday.
If you set off on a diversion and have to declare Mayday straight away, then something has gone wrong. Either the crew held for too long and have started the diversion below the normal reserves or the fuel planning was wrong, by which I mean maybe an unrealistic route from destination to alternate was catered for.