I saw this mug to buy on Amazon
You Bet Giraffe I ❤ Giraffes
My question is, how does a native speaker interprets this sentence?
(Edit) Actually "You bet giraffe" is a play on the common expression "You bet your ass" which is an enthusiastic and slightly vulgar positive response to a question. "Giraffe" sounds like "your ass", just more acceptable in a polite context.
Do I like giraffes? You bet your ass I do!
So in this case:
You bet your ass I ❤ giraffes
I'm going to leave my previous answer below as general information, but be aware I completely missed the joke.
"You bet!" is an enthusiastic positive response to a yes/no question:
Do I like giraffes? You bet I like giraffes!
Would I ask a giraffe to a formal dinner! You bet!
The expression on the mug is not complete since there is no question. Also there would ordinarily be a comma between "You bet" and "giraffe" to indicate you are speaking directly to the giraffe. For example, suppose the giraffe asked asked me for something:
Could I bake us a cake to go with our afternoon tea? You bet, Giraffe!
Note that I capitalize "Giraffe" because it's not uncommon to name an animal what it is, if the animal doesn't have its own unique name. Also, depending on how well I know the giraffe, I might call him "Mister Giraffe" to be more polite.
Good morning Mister Giraffe! Looks like rain today, don't you think? Although you would know better than I, wouldn't you, with your head so much closer to the clouds than mine, ha ha.
Obviously, giraffes can't talk, so this is a fanciful situation, but pretty normal for things like children's stories.
This is a roundabout way to say the expression on this mug is not entirely idiomatic -- but since you asked how a native speaker would interpret it: I would assume an unspoken question, and mentally fill in the missing punctuation.
(Hello Giraffe. Do I love giraffes?) You bet, Giraffe! I ❤ giraffes!
It's a pun on the expression You bet your ass... (ODO):
you bet your ass
You can be very sure.
[with clause] ‘you can bet your ass I'll go for it every time’
At least in AmE, when speaking quickly, your can sound (roughly) like jer (the same j sound in the word jury). In other words, your and the gir in giraffe can sound similar. Phonetically (roughly), it is:
You betjeraffe I love giraffes
→You betjer ass I love giraffes
→ You can be very sure I love giraffes