A dark pattern is an element in a user interface that’s designed to manipulate users to act in a way that they don’t intend to, often by manipulating how users navigate or confusing them about what their options are.
I'm not sure if this interface is a dark pattern so much as it's a social assurance that a 5% tip is acceptable. We're all used to seeing recommended tips in coffee shops or at restaurants, and lots of us decide on a flat rate of $1/drink or 20% of the total ticket, whichever is higher. I'd wager that grocery delivery is such a niche service that people don't have an expectation of what they should tip. As soon as the company says, "You should tip 5%!", that becomes the norm for users.
It's not a manipulation so much as it's a misdirection. I think that still qualifies as a dark pattern, but either way it obviously doesn't change how shitty Instacart is to its employees*