Pierce was definitely a better player than Garnett. Garnett proved during his tenure at Minnesota he couldn't carry a team on his own. Missed the playoffs 3 years in a row during his prime. That's not superstar material.
Pierce, of course, had the advantage of playing in the height of the (L)Eastern Conference, but we won't talk about that.
In reality land, there was no point in NBA history where Paul Pierce was actually a better player than KG prior to KG's 2009 knee injury, and that has nothing to do with respect or disrespect -- it's just factual. Love Pierce, Love KG, KG was the better player. End of story. It's like saying Russell was a better player than Parish: it's not a slight, just a fact.
It's summer league. He should be dominating it.
Absolutely. Legitimate NBA players crush Summer league the same way they destroy the D-League. It's probably worth pointing out that Fab Melo destroyed in the D-League.
However: and to bring it full circle: I don't think any of us* expect Marcus Smart to be the next Paul Pierce-level talent on the Celtics. That doesn't mean he can't be a solid starter on a playoff team and can't be a player worth holding onto for his tenure in the NBA.
*I say any of us, but I mean people thinking reasonably, not the people that wouldn't trade Smart for Kevin Durant et. al. That's just utterly indefensible if you lay any claim to objective thinking.