The shirt actually says "Bananas are snacks", not "Bananas are so yummy".
This is a reference to an famous old meme about elementary school field trips, where the teacher will tell the students that they should bring snacks with them (usually up to a maximum value, like 500 yen) and then a student asks "Is a banana considered a snack?"
This is apparently a common enough thing that a news website did a poll on twitter with the question "Did the policy of you or your child's elementary school count bananas as valid snacks?".
Out of 348 votes:
7.5% - "Bananas are consider snacks"
34.2% - "Bananas are not considered snacks"
58.3% - "The question wasn't asked, or I don't remember"