Firstly, in the books so far, GRRM has only used a few words of Valyrian. When it got turned into a TV show, David Peterson used those few words to create the language for the show. Indeed, it turns out...
Yes, it's a reference
Not only is 'Oa mysa iles me nýnyghi.' Astapori Valyrian for 'Your mother is a hamster', but according to this interview with David Peterson, all the Meereenese rider's insults are a reference to that scene from Monty Python and the Holy Grail:
He's [the Meereenese rider] actually saying a Low Valyrian translation of the French guy's insults in 'Monty Python and the Holy Grail.' That was [series creator] Dan Weiss's idea and it was so hilarious that I had to do it.
Furthermore, there's an online dictionary and within that we find that, in the season 4, episode 3 Astapori Valyrian dialogue the insults translated in full:
- Champion: Oa mysa iles me nýnyghi, si oa kiba tuziles espo tomistos!
“Your mother was a hamster, and your father smelt of elderberries!”
- Champion: Já si hojgá oa gundja, trezy eme mero dovodedha! “Go and boil your bottom, son of a silly person!”
- Champion: Kiman nya másina orvorta va oi sodjistos! “I wave my private parts at your aunties!”
- Champion: Do eban av kimívagho dombo, o doru-borto pame espo gruzi evi havor espo begistos! “I don’t want to talk to you no more you empty-headed animal food trough wiper!”
- Champion: Ghorgan ji pungo va o, nynta Dare espo Zaldrizes, o si une oi dovodedhi, Vesterozi azzzzzantys. “I blow my nose at you, so-called Dragon Queen, you and all your silly Westerosi kaniggets!”
As for other references to Monty Python
Well... Let's start with the word Hamster... According to this post on his blog, the word 'nýnyghi' "may have been inspired by the Knights who say Ni".
Are there even more references? I don't know of any more of the top of my head, but David Peterson does occasionally put references in here and there, so it's not impossible.