I’m not giving access to my Mastodon account to some random service I’ve never heard of for no reason.

If it makes you feel better, it's all client-side: there's nothing executing on the server (I'm running a copy of it on a server that just... can't execute anything) so it's not doing any data stealing.

Buuut, since it's trivial to host, you could grab a copy of the code and host it yourself as well.

It's a nice user interface, made by @cheeaun@mastodon.social, and loved by a lot of people.

If you don't want to trust people with your account details that's fine - then using a third party app is probably not for you, unless you're willing to either trust people or dig into the source code.