Clearly there's an axis with security on one side and convenience on the other. Tilting fully to either side results in a terrible experience. For me, Telegram strikes the perfect balance. Secure chats have fantastic privacy, while the UX isn't totally garbage. The trade-off is that chats are non-E2EE by default.
Signal's has the axis tilted all the way to the "security" end, so that the product is unusable if you aren't a technical expert already.
Could your mother figure out how to encrypt an email message using GnuPG? Absolutely not a chance.
Could she figure out how to send a message on ProtonMail that encrypts it client-side for her? Probably.
Signal is GnuPG. Telegram is ProtonMail.