Is macOS Monterey 12.7.6 still good for iMac?

Monterey is mid-life. It is no longer receiving security updates and this may be the last year it supports big-name 3rd party software such as Adobe Cloud or TurboTax. However third party web browser support is still strong and old versions of most software will continue to run fine.

If your Mac can support a newer OS you should probably upgrade. Otherwise Monterey is still good enough for the average user (assuming you don't need the 2025+ edition of big-name software).

I would upgrade to a more current macOS if possible.

The model year of an iMac is available at Apple menu > About This Mac.

A late 2015 or mid 2015 iMac can be upgraded to the latest version of Monterey macOS 12 but no later. 

A 2017 iMac can be upgraded to the latest version of Ventura macOS 13 but no later. 

For all of the above, see instructions in: How to download and install macOS - Apple Support

A 2019 or later iMac can be upgraded to the current version of Sequoia macOS 15. See: How to upgrade to macOS Sequoia - Apple Support

The following has information about when 12.7.6 was released and about security releases since then: Apple security releases - Apple Support

Already does not support TurboTax (only Ventura, Sonoma and Sequoia).

My 2019 27" iMac uses Monterey and no issues. I keep that version because I have some programs that are not compatible with newer versions.