Let's say for a moment that you are telling the truth. No one will counterclaim because if they did, they'd be admitting they are mafia.
Let's say for a moment that you are lying. We don't know if this game even has a miller to counterclaim you (miller's not something that's in every game like a cop or a doc). So yeah, no counterclaim =/= you aren't mafia.
The sucky thing about millers is that you are likely to be lynched or vigkilled at some point in the game, even if you claim day 1.