Well, this is a really tough one. Since Omegle is a platform where you can randomly meet people, running into your mom and having a nude - related situation is just horrible. You could start by apologizing if you were in any way involved in inappropriate behavior. And for your mom, she should also be more careful with her online activities. You both need to respect each other's privacy and set clear boundaries about online usage.
That's an extremely awkward and inappropriate situation. First, you should immediately end the interaction on Omegle. Then, it might be a very difficult conversation, but you may need to talk to your mom about the importance of privacy and appropriate online behavior. Try to approach it calmly and not accusatorily.