is it still safe to use iPad mini 2?

Yes. The current iOS 12.5.2 for it has the security patches and it was released a month ago.

link-> About the security content of iOS 12.5.2 - Apple Support

There have been no reports of vulnerabilities or other exploits in iOS 12 beyond that.

And even if there were, unless you are a target for hackers its unlikely they'd take advantage of such things on 8 year old devices.

A first generation iPad Air, iPad mini2 and mini3 can only be updated to iOS 12.5.2.

Apple ended major update support for these models of iPad in September 2019 - although they since continue to receive “point” security updates, the most recent being just a few weeks ago. These devices cannot be updated to iPadOS 13 (or any later major versions of iPadOS) as the internal hardware does not meet the minimum technical requirements (CPU and RAM) for newer versions of iOS/iPadOS.

Some App developers have dropped support for iOS versions preceding iOS 13.x for technical reasons (often because APIs or software libraries upon which they rely have been updated in the new version of iPadOS, making their App incompatible with earlier versions). 

Whilst many App titles remain compatible with your iPad, if you want or need to access to Apps requiring the most recent versions of iOS, you’ll need to consider replacement of your iPad with a newer or current model that supports iPadOS14.

As for “safety” to use, perhaps consider that the iPad mini2 is very mush in the “twilight” of support. You would be well advised to consider replacement with a newer model sooner rather than later.

I hope this information provides helpful clarity as to your available options - and the limitations of your existing iPad.

Apple HAS been continuing to support iOS 12 devices with security patches.

Hence why iOS 12 is currently at iOS 12.5.2.

It is still safe to use that 2013 iPad Mini 2.