Is it okay to not update iOS on a Mac?

In general, it's a very good idea to keep your Mac updated as recommended by Apple. Updates usually include needed security improvements. Sometimes, that is the major purpose – the 26.7.1 and 27.0.1 updates are mainly a patch for a zero-day exploit thought by Apple to potentially be in the wild.

To be clear, there is a difference between an upgrade and an update. Upgrade means a major version change, e.g. macOS 26 Tahoe to macOS 27 Golden Gate. That is optional, assuming your hardware supports it. An update is within a version, e.g, 27.0 to 27.0.1 or 27.0.1 to 27.2. If you remain on an older version, Apple generally updates those with security patches as needed for several years (in some cases longer, for example Apple earlier this year Apple released point updates for Catalina and Big Sur, versions of macOS that launched in 2019 and 2020, respectively).

So, no need to upgrade to Golden Gate if you don't want to. Note that some major applications (MS365, Adobe, TurboTax, etc.), support only the most recent three versions of macOS, meaning that if you use those apps you will need to update...eventually.

But you should keep your Mac updated on the current major version. So, if you are running Tahoe then current version of macOS is 26.7.1.

See: Update macOS on Mac - Apple Support

For the moment. Fewer application updates and websites will remain compatible with Tahoe over time, and once macOS 29 comes out, it may stop getting security patches.

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You can't update or upgrade iOS on a Mac at all since Macs run macOS. iOS is for Apple's iPad and iPhone mobile devices.

But no, you don't need to upgrade to macOS 27 if you don't want to. As Niel noted, macOS 27 (Golden Gate) just came out and Tahoe will continue to get security and app updates for about three years yet.