Yes, Clean Monday is the start of Orthodox Lent, roughly equivalent to Shrove Tuesday. It is a national holiday and people normally go out and eat fish so fish tavernas are typically packed.
No, I haven't been to any big carnival processions. I did go to the one in Kalyves many years ago but that is quite small.
As an aside, abstinence/fasting in the Orthodox church is longer than most people think. Clean Monday this year was on 15 March and Easter Sunday, Pascha, is 2 May. That means from Clean Monday to Easter Saturday is 48 days all of which is fasting. The usually quoted 40 days runs from Clean Monday to the Friday before Good Friday and is meant to commemorate the 40 days Jesus spent in the wilderness. The Saturday is Lazarus Saturday and the Sunday represents Palm Sunday and are also both fast days. The following week is Holy Week representing Jesus' trial, torture and execution ending in his resurrection at midnight on Saturday so again all fasting
I doubt if many Greeks, other than monks and priests obey the full rules which involve both fasting, i.e. not eating between certain times of day, and abstinence, i.e. not eating certain proscribed food groups. Many Greeks obey abstinence rules for the whole period often quite rigorously. A lot are a bit more lax but do partially observe abstinence rules for the whole period or completely in Holy Week. Some only obey abstinence/fasting on Good Friday and Easter Saturday.
Apologies to religious scholars if my description is not totally accurate.