Android backs up data to the cloud. If the phone breaks or gets stolen, you don’t need to recover data from it - you can just pull it from Google’s servers.
In addition, people tend to not treat their phones as “permanent storage”. The concept of losing or breaking their phone is probably more clear, so they make sure to back it up in some way to the cloud or their desktop.
Also, it’s much more likely for a phone to be stolen than a laptop or desktop.
It’s possible for an upgrade to break things and leave your system in an unusable state or cause your data to be lost.
However, that could happen at any time with no warning. Your hard drive could break, your charger could cause a short, your laptop could get stolen. If you have any files you don’t want to lose, I’d strongly recommend you set up a backup asap.
In terms of whether to actually upgrade, Mint 20.3 stops receiving security updates in April so you should probably upgrade to 21 sometime before then.