Is a new GitHub account a red flag when looking for jobs?

A new account isn't necessarily a red flag. You could use it as a professional portfolio and put a few relevant projects there. If someone asks why it's new, you can simply explain that you recently set it up for your portfolio.

Make sure you have the rights to publish anything you move or copy over, and check GitHub's current account policies before creating another account.

You could keep the existing account and curate what you show. Link employers directly to the projects you're most comfortable discussing, and review your profile for anything you don't want to highlight.

That doesn't necessarily hide your other public contributions, though. If separation is the main goal, a separate portfolio may feel more comfortable.

I wouldn't assume the subject matter makes your work worthless. An unusual project can still demonstrate useful skills, and you don't need to be ashamed of a hobby project just because it isn't conventional.

That said, employers may react differently, so consider whether you're comfortable having it come up in an interview.

For a link you're sending to recruiters, keep the presentation relevant to the jobs you're applying for. A small collection of clear projects is easier to assess than an account where the work you want noticed is buried among unrelated material.