I would say depending on who is asking you...
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.