[MENTION=6670929]Havrik Stoneskimmer[/MENTION]
Was this furrowed eyebrows and glares at you of the chummy "damn you DM" variety? Or was it genuine game interfering anger? If the latter, that seems an unusual reaction for players to have...unless there's more to this story...
What were circumstances leading up to and during fight? Any other reasons why they might be upset?
It sounds like your party is the classic fighter, thief, cleric, and mage. In which case during a fight against a flying foe the fighter and thief are *probably* going to have a harder time finding effective ranged attacks. Fortunately if that's true there are easy things you can do as DM to give them more options.
Has this group ever been trounced bad like this before? Has the play style up to this point favored the PCs such that failure or TPK wasn't a real possibility?
Have they ever dealt with a fleeing enemy before? For example, if you fudged something in the past to help a villain escape they might hold a grudge and suspect you of "cheating."
Have they ever fought a dragon in the open before? That's probably one of the most important points, especially as this was their first solo fight. Dragons are fearsome opponents. Green dragons, in particular, are cunning deceivers who never fight fair and try to set up pitches battles - it's their nature.
IMO a good fight with a green dragon should...
Have at some point a dialogue with the dragon so it can tempt one or more PCs, setting up a betrayal by an NPC, or tricking the PCs into a death trap and gloating/sword-point negotiating with them.
Use of flyby attacks that channel PCs toward a dangerous obstacle, flushing them from cover like rabbits, even grabbing PCs to carry them up and drop them...preferably among it's minions who can stabbity stab, or on top of another PC
Ideally set up a complicated situation where the PC become unwitting architects of their own doom...at least at first. For example, the PCs in order to pass through a dungeon room filled with dense poison gas activate drain gates which allow the gas to seep out of the room into caverns below. And the caverns below just happen to be the green dragon's lair and treasure horde. Oh wait it resists poison! And the gas is dense so it clings just above head height. Poor adventurers.
And finally provide a character flaw for the dragon the PCs can exploit. For example, many green dragons can't resist a challenge of wits like a riddle contest or a deadly game of hide and seek.
Btw, I usually tweak dragons to give them extra powers to avoid stun lock, to take advantage of their grab attacks, to change their resistances into something more like the volcanic dragon, make them scarier when bloodied, and give them a signature "predatory" power by color. In a word, my version of your fight *probably* would have been tougher.