Plots that would be solved immediately if J'onn's mindreading always worked

Oh yeah.....how come J'onn didn't just mind read Mon El when they first picked him up? Oh wait....J'onn can't read Kryptonians. Must be the same for Daxamites?

Honestly, though, this is why I don't mind Kara getting her butt kicked from time-to-time. If they truly portrayed her as being the strongest, obsolete superhero, for me that would take away from some of the suspence/drama.

I also like it when the heroes face adversity. But it gets kinda frustrating if their powers are handled so inconsistently. Maybe they could have written something in to explain why J'onn's powers don't work consistently (maybe the white martian blood infection). But now one episode (Distant Sun) he can take on a powerful telepath, and the next (Alex) he can't even read normal humans.

If J'onn was worried that his powers don't work reliably that could also explain why he isn't in the field more often.

Or they could have turned it into a moral thing where he doesn't like to read people's mind without their permission unless it is really necessary, but they clearly didn't go with that, since he has read both friends and foes without permission on the show.