Nothing actually wrong with it as such, other than the customer may feel that if they ever want to replace it, or it needs to be removed for repair/new floor etc., they would then need an electrician to disconnect it.
I know that everyone has their own preferences, but I like to keep sockets in the fabric of the building, so will put the socket either behind the appliance (if there's room) with an isolator above the worktop, or behind an adjacent cupboard and cut a hole in the back for access. I realise these options weren't open to you and you did what was necessary, so you could always appease the customer by offering to disconnect it for free if that's ever required.