It can be broken down/understood this way:
You think its funny.
This could be a neutral statement, or a criticism.
In this case it feels like a criticism.
Notice the phrase "You think". Not it is funny or isn't, but "you think it is". They are drawing attention to the other person's belief/interpretation of it, not discussing the subject itself, or whether it is actually funny or not.
This is very common in English writing and speech, "You think its a good idea" where I appear to differ, implies strongly "I disagree with your conclusion".
So the implication of this as a bare statement, would probably be that the speaker doesn't agree and is criticising the other person: "You think it's funny (but I don't)."
I'm glad you think (something).
When you are clearly not glad or impressed at what the other person does, or thinks, but you say you are glad they did this stupid or wrong thing, that's sarcasm. It emphasises the criticism and disapproval. There is almost no other interpretation of this structure.
"I like how you support the government even though they failed on COVID" - means more likely, I don't like the support, I think you are wrong/stupid/ignorant, and I want you to know exactly that's how I feel.
Putting these together:
I'm glad you think its funny.
Depending if the 2nd part is criticism or not, the statement is sarcastic or not. It's much more likely the 2nd part is criticism, from the context, so its very likely this is sarcasm.
But I want to emphasise that it could be used both ways, in everyday life. Compare these:
- "So you spent our child's food money for the week on alcohol? I'm glad you think its a good idea." - sharply underlining disapproval by using sarcasm.
- "So you're applying for jobs after all? I'm glad you think it's a good idea." - could be genuine or sarcasm, hard to tell.