Lamb hasn't shown that he's an above average hitter. 87 wRC+ for a 3B is terrible. Castillo is a tiny bit above average for a catcher, Tomas hasn't shown he's an above average hitter, 88 wRC+ for an offensive first player isn't good.
Now of course Lamb and Tomas could get better. It's hardly impossible and probably likely that they will get better. But it's not something you just say they're good.
And I woulnd't bet too hard on Peralta.
May be not above average at their respective positions, but overall they should be better than the average major league hitter. And for the purposes of lineup construction, I don't think their defensive position is all that relevant. Lamb is a .700-.750 OPS guy. Certainly not terrific when adjusting for parks, but not terrible either. Tomas was a rookie and held his own. I'd expect some improvement from him.