At some point in the career of every action movie star, they'll try to step outside their comfort zone and do a comedy, or a drama. For some actors - Bruce Willis, for example - the transition goes very well. For others, like Sylvester Stallone, it's an unmitigated disaster. 'Stop, Or My Mom Will Shoot' was that disaster, and Stallone is in no doubt whatsoever about how terrible it was.
When asked for his opinion on it in the past, Stallone felt like calling it the worst movie in the history of the world wasn't a sufficient description of its awfulness. He went a step further, stating that it was likely to be the worst film ever made in the history of our solar system, including anything made my aliens that we haven't had a chance to see yet. That's a pretty big statement!