What do you base this on? He's a Marxist with completely different views and priorities to the EU negotiators. Do you know of actual "stupid" things he's said or done, or are you just buying into the EU/German/banker line because it's the easiest?
Call the other negotiating part terrorists is stupid. How he talked about the referendum. His views being different or not you have to realist.
The referendum to me was not very well thought through. They asked a question about two complex documents that amounted to: Would you like to feel less financial pain? The answer was obvious.
I think you can make a case for calling them terrorists in the context he used it in. Obviously he feels the Greeks are being held to ransom by people threatening financial meltdown if Greece doesn't to what they say. Calling them terrorists might be emotive language, but it's not all that far-fetched imo.
On the referendum question, we don't know if he was involved in wording it, and in any case I don't really agree that it can be boiled down to "would you like to feel less financial pain?".