Donations are a common way to get some extra money for your website to pay for hosting fees and such, but a standard donation button could be improved upon.
I have seen a lot of websites write "Buy me a coffee" and set the donation amount to like $3.00 or something around the price of a coffee.
I think this is a pretty nice way to soften up a donation button , but if it was me since, I'm crazy for design I'd have to do something like "Buy me a shot of Tequila".
Note: Design portfolios should not have any sort of advertisements. It should be nothing, but your work in my opinion and usually hosting costs are low enough that it doesn't matter anyways.