What does "goo" mean in this context?
Turns out it can't. Schumer herself is lively and likable throughout the film, but she's unable to surmount its puddle-of-goo ending, which is set at a company launch party at which Renee delivers a triumphant address to the many women on hand.

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Ice cream when it melts goes gooey.  Its a general word for a melted thick substance that is usually sticky.

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To add onto what Bill said, in this context they're saying that the ending essentially "melted into a puddle of goo", in that it fell apart and wasn't very good. It couldn't hold its structure, and once something has melted you can't really get it back.