No. This is caused by a problem with the SSL configuration of the azduino.com website from which the ATTinyCore:micronucleus@2.5-azd1b tool dependency of ATTinyCore is downloaded:
$ curl https://azduino.com
curl: (35) schannel: next InitializeSecurityContext failed: SEC_E_CERT_EXPIRED (0x80090328) - The received certificate has expired.
It affects everyone universally rather than being a problem specific to your own system. It has been reported to DrAzzy/SpenceKonde here:
The workaround will be to manually download the tool archive (the archive download URLs can be extracted from the package index file) and then place it in the Boards Manager download cache. Arduino IDE checks for the file in the cache before attempting to download it from the Internet, so if you have manually placed the file in that location, you avoid the problematic download that causes the ATTinyCore installation to fail.
1. Download the ATTinyCore:micronucleus@2.5-azd1b Archive File
Unfortunately you will likely find that attempting to download this file via the browser is not very user friendly. For security reasons, the browser will try to strongly discourage from downloading it and you will have to make some efforts to overcome its resistance.
This is the download URL:
If you are using Windows:
http://azduino.com/bin/micronucleus/micronucleus-cli-2.5-azd1b-x86_64-mingw32.zip
If you are using Linux x86:
http://azduino.com/bin/micronucleus/micronucleus-cli-2.5-azd1-x86_64-linux-gnu.tar.bz2
If you are using Linux ARM:
http://azduino.com/bin/micronucleus/micronucleus-cli-2.5-azd1-arm-linux-gnueabihf.tar.bz2
If you are using macOS:
http://azduino.com/bin/micronucleus/micronucleus-cli-2.5-azd1-x86_64-apple-darwin.tar.bz2
2. Add the Downloaded File to the Boards Manager Cache
After you manage to download the file, copy it to the following folder:
If you are using Windows:
C:\Users\<username>\AppData\Local\Arduino15\staging\packages\
(Where <username> is your Windows username)
If you are using Linux:
/home/<username>/.arduino15/staging/packages/
(Where <username> is your Linux username)
If you are using macOS:
/Users/<username>/Library/Arduino15/staging/packages/
(Where <username> is your macOS username)
3. Install ATTinyCore
Now try installing ATTinyCore via the Arduino IDE Boards Manager again. The installation should now be successful.