Yes and no. Yes in that it’s a different shell that is intended to be used instead of something like Bash, and isn’t compatible with Bash scripts. However, unlike Fish and Zsh, it’s also not compatible with plain POSIX sh, so you have to run plain shell scripts by calling into something else that is sh-compatible.
No, it is its own scripting language.
Is nushell comparable to fish?
Yes and no. Yes in that it’s a different shell that is intended to be used instead of something like Bash, and isn’t compatible with Bash scripts. However, unlike Fish and Zsh, it’s also not compatible with plain POSIX sh, so you have to run plain shell scripts by calling into something else that is sh-compatible.