At Schiphol is 55 minutes enough for a transfer to an EasyJet flight?

It's extremely short and probably impossible, even with no hold luggage. Easyjet has no interline agreement with other airlines, which means not only that they would not take care of any hold luggage for you but also that they don't offer any facilities for transfer whatsoever and that the answers to earlier questions about transfer in Amsterdam do not apply. They will simply treat you as if you came from outside the airport and expect you to be there well in advance for security and boarding formalities.

Easyjet flights depart from the low-cost pier, which is a bit off the way, has long corridors and frequently long lines for security. Delta flights mostly dock at D-gates, in departure hall 2, so you will have to walk to departure hall 3 and then some more to the low-cost pier (but contrary to what I wrote in an earlier version of my answer, you should be able to do that airside and avoid waiting for a passport check).

Because Easyjet only operates point-to-point flights, they also won't help you in any way if you miss your plane. If you have any delay and fail to show up at the gate in time, you will be treated as a no-show and simply forgo the benefit of your ticket. At most, they might allow you to change to a later flight (I had to do this one time that security was particularly bad, and the fee was €50).

On the other hand, boarding passes can be printed 30 days in advance (the default is automatic online check-in, not airport check-in, I am not sure whether the latter is still possible at all) so at least you don't need to worry about that. Otherwise, you would need to queue again at Easyjet departure desks, with all the passengers starting in Amsterdam (no transfer desk either). Because I always check-in online, I don't know how long this typically takes but I have seen long lines there as well.

It may actually work, though I would advise against it:

• in case of any delay on your arriving flight, Easyjet will not help you.
• depending on your seat location, it may take quite a while before you can exit the Atl-Ams plane.
• touchdown may be in time, but if you land on the Polderbaan runway, the plane will have to taxi about 15 mins before reaching the terminal.

But, in contrast to Relaxed's answer, you may actually be able to stay on the transfer side of the airport terminal:

• have your Easyjet tickets printed beforehand.
• Gatwick, UK, is not part of Schengen (Britain has opted out of this). As a result, on Ams airport, flights to the UK leave from the same part of the airport as international flights. Same part is a lose description: you may still have to walk quite a long way, but in principle you shouldn't have to cross any passport control, in or out of Schengen. You just stay in the passenger transfer area.

Easyjet flights often leave from a separate terminal (gates H/M)), which in the past has had its own security check, but I think that has changed in recent times and you may not encounter any security check between your arrival and deparate gate. This terminal tends to operate as an international, non-Schengen terminal; you should already be on that side of the airport.
But Schiphol terminal tends to be rather in flux, so that may have changed by the time you fly. tripadvisor has this to say (most recent edit Aug 2015):

From June 2015 new security measures aimed at making transiting Schiphol even easier were introduced. Now there will be no security checks at the gate as was the previous situation. For passengers arriving in the non-Schengen area of the airport from countries that are considered to have carried out a recognised level of security checks such as the USA and all NON Schnegen EU countres (e.g. UK), no further security screening will take place.

(Sidenote: the actual recommended transfer time given for international to EU flights in Ams is 40 minutes, though I only find unofficial quotes: the schiphol.nl website appears not very practical for finding the kind of information you need. Note that "EU flights" here is not the same as a Schengen flight: UK flights are EU flights and in general have shorter boarding times compared to e.g. USA flights from Ams, hence the 40 mins transfer time.)

I cannot tell the exact answer but there is some information that may help you.

1. 55 mins is quite tight for transit, and you may have to wait longer for transit check on the last day of Christmas holiday.

2. Easyjet won't help you if you miss the flight.

3. Base on information I guess you first leg is DL(Delta) 74. Although the flight route is MIA-ATL-AMS , two legs are operated by different planes. MIA-ATL leg delay will not affect you flight; however, the leg ATL-AMS sometimes delay more than 30mins.

4. The 2nd leg U2(easyjet) 8874 usually delays more than 30mins , the delay is usually result of longer boarding time or delay of earlier flight.

I'd advice against it.

Schiphol airport is a fairly large airport, and walking times between different gates can be quite long. EasyJet generally departs from the H/M gates, which are quite a bit out of the way. International flights quite often arrive at the Polderbaan, which is quite the taxi distance from the pier proper.

If you arrive at the polderbaan, 15 minutes of taxing to the gate is no exception

If you depart from Gate M, 15 minutes of walking to the gate is no exception

If you are lucky, you might make it, but it's not something I would advice anyone to do.