This is an interesting question, and as you can see you've gotten a bunch of different responses already.
The only real answer I can give is: it depends.
To me, personally, obviously silly or in-character "trash talk" directed at the enemy team is fine, as long as you're not doing it so much that it becomes spam. (I would hope you also perhaps say some things that aren't solely trash talk, even if it's only "glhf" and "ggwp".) There is -- again, IMO -- a big difference between "/all DRAAAAAAAVEN!!!" after a clutch play as Draven and "/all HAHAHAHA YOU FKIN SCRUBS SHUD UNINSTALL AND KILL URSELVES" after the same play. I would hope that people can appreciate the former to some extent, but most players are not going to take the latter very well. (Nor should they.)
Relevant points from the Summoner's Code (http://gameinfo.na.leagueoflegends.com/en/game-info/get-started/summoners-code/):
Enjoy Yourself, but not at Anyone Else's Expense
We want you to get excited, to have tension-filled moments, and to celebrate your success. This doesn't mean that we're okay with you ruining anybody else's day.
(It goes on to talk about how it's bad to attack the players on your team, but makes no specific mention of trash-talk directed at enemies.)
Could trash talk "ruin someone else's day"? Perhaps. Someone might take it the wrong way -- but anyone might take anything the wrong way, which is why chat restrictions and punishments have been based on the community's standards and not the standards of the most sensitive players. Still, you run the risk of seriously pissing someone off, especially if you're trying to play head games and not just be funny.
Show Humility in Victory, and Grace in Defeat
While it's alright to celebrate, make sure that you keep any gloating (or any other mode of self-indulgence) out of all chat. Instead, thank your opponents for the game. After all, despite their best efforts, they just made you a very happy person.
Trash-talking in /all chat after after a kill or if your team is dominating is, well, gloating. If you're being so silly or over-the-top or obviously RPing, it's less likely to be taken as you seriously insulting the enemy team, especially if you are a gracious winner/loser at the end of the match.
An alternative standard that Riot stated back when the Tribunal was active was "is this player being a jerk?" and/or "would I want this player on my team/in my game?". If you're actively attempting to piss people off, you're likely to eventually end up on the wrong wide of those. It's a fine line between "getting under their skin but keeping it light-hearted" and "being a huge jerk".
"You smell like burning"
"Mundo think you a big sissy"
"Let me sing you the song of my people"
"Ok"
