If you've come across mumon-b--cdn-net.cdn.ampproject.org and wondered whether it's safe, the short answer is yes. This is a Google-hosted endpoint used to serve AMP content, part of Google's cdn.ampproject.org infrastructure. It's not a standalone website or business, but a technical service operated by one of the most reputable companies in the world.
The domain has a strong security setup: a valid SSL certificate, enforced HTTPS, and no history of malware or phishing. It's hosted on Google's own servers and appears on no blacklists. The lack of an about page, contact info, or privacy policy is expected here — this isn't a consumer-facing site like an online store. Think of it more like a delivery service for cached web pages.
In short, there's no reason to treat mumon-b--cdn-net.cdn.ampproject.org as suspicious. It's a legitimate part of Google's content distribution network. If you're seeing this domain in your browser's address bar, it's simply because you're viewing an AMP page that Google is accelerating.