The common thought process is that because the Greeks delayed Hitler, operation Barbarossa was delayed and thus the Germans got caught in the winter weather and were beaten back from Moscow. This was a very common idea in books about WW2 in 60s-70s, but it's completely false. First off the Germans delayed Barbarossa for two reasons, firstly because every spring the Western portions of the Soviet Union would suffer from massive rainfall that would turn the roads and fields into mush, obviously that would have caused havoc for the Germans tanks. Secondly the Germans were having trouble building the necessary infrastructure/airfields to support all their troops in Poland. Furthermore, the Greeks only really cost Hitler a few weeks at most, a few extra weeks wouldn't have made a difference for the Germans.

Now this isn't to say that the Greeks don't deserve any credit, they fought bravely and heroically, and certainly punched aboard their weight; especially with regards to Italy where the Greek troops dealt serious blows to the Italian army and almost pushed the Italians out of Albania (which had been part of Italy since 1939). They also fought heroically on the island of Crete, where they fought so well agains German paratroopers that Hitler vowed never to do another large scale airborne invasion again.

The impact of Operation Marita (the invasion of Greece) upon Barbarossa is topic in which there is a degree of contention amongst historians. A number of German memoirs from the 1950s and 60s asserted that Marita delayed Barbarossa by vital weeks. This was part of a general trend among German memoirists to push an exculpatory historiography in which the fault for German defeat lies at anywhere else but their own feet. However, as /u/Sid_Burn and other historians have pointed out, a few weeks alone would have not cost the Germans the campaign. David Glantz has pointed out that Marita and Punishment (the invasion of Yugoslavia) only used a portion of the Barbarossa force structure and they accomplished their goals with little cost save one exception. Marita may have even helped Barbarossa by delaying the Soviet invasion because there is some evidence that the Germans wanted to launch it on 15 May. Soviet intelligence had given Stalin this earlier date as an invasion date and the failure of the Germans to invade on 15 May made it easier for Stalin to ignore his intelligence reports about an imminent German invasion.

However, the occupation of the Balkans proved to be a net drain for the Third Reich, which as the war ground on became ever more short of manpower. The invasion of Crete made the Germans reluctant to engage in future airmobile operations, something which they were more willing to experiment with in the earlier campaigns. And the Greek resistance also was yet another nail in the coffin for the reputation of Germany's most important ally, Italy.

Sources

Citino, Robert Michael. Death of the Wehrmacht: The German Campaigns of 1942. Lawrence: University Press of Kansas, 2007.

Glantz, David M. Operation Barbarossa Hitler's Invasion of Russia, 1941. Stroud: History Press, 2011.