To someone with full blown TDS, sure, a Snowden pardon probably won't sway your opinion at all. But think of someone who leans libertarian and maybe doesn't love Trump but also doesn't hate him, and remembers how the Obama admin forced Snowden to flee and revoked his passport, forcing him to stay in Russia. Seeing a pardon from Trump could move them into being a mild supporter.

A lot of people have one or two big issues that their political allegiance revolves around. It can be abortion rights, gun rights, taxes, the national debt, anything. If someone's core issue is fighting the surveillance state, a Snowden pardon would be a massive change in the landscape.