If you have there some xmr valued many hundreds of euros, it is pretty unlikely you actually mined it unless you have some very powerful setup.
If I am the government, then I will ask you: Show me this mining setup. Prove to me that you at least owned or had access to this setup.
I think when we talk about plausible deniability it's not in the context or "Where did this money come from", but more along the lines of "I cannot prove it was actually you who bought this item".
Imagine you buy an item that is restricted or sanctioned in your area, plausible deniability makes it so it is very difficult for me to prove in a court it was you who bought it, and very easy for you to evade these accusations.
If someone forces you to open your wallet, you can just have a second hidden wallet where you keep your stash, and then just open another wallet with, let's say, 5 or 10 euros worth or XMR that you can actually mine in a reasonable amount of time with some old machines.
I would like to add that if you actually have XMR in a wallet that you got from mining, and you never moved those funds before, you CAN confirm that those funds came from mining. In fact, the GUI wallet will show a symbol indicating that it came from mining (what's called a "coinbase" transaction).
However you could just say that you moved the funds from the mining wallet to this other wallet, but then I will ask again: Show me the mining wallet that got those mining transactions.
if i do that, pretend i move the mined XMR to my now NEW wallet and you ask " Show me the mining wallet that got those mining transactions.", can i Just say i dont have the mining wallet anymore coz my computer got infected so i move all my xmr to new wallet and forget the old one alltogether?> Show me the mining wallet that got those mining transactions.
thats actually genius, you can just say that your computer got infected before, and move all the funds to new wallet and burned the old one just to be all safe then it cant be track anymore? is that what you trying to say?
what if I say that I mine it when monero is at low value, then it just get appreciated overtime and become high value... and when they ask me to see the setup, just say that you dont have it anymore coz it was long time ago? is that still work? > me the mining wallet that got those mining transactions.