An admission of jealousy. "I'm so jealous". An adaptation of the phrase "miso horny", which is a mutation of the phrase "me so horny", which is a poorly translated version of the phrase "I'm so horny". But with jealousy instead of horn.
Miso jelly could also conceivably be a type of food, which facilitates humourous use of the phrase as a pun.
"OMG you got the latest iButtPlug, MISO JELLY"
"What's the matter baby? Can't finish your supersized bowl of miso jelly?"