Luke seems to be wary of directly interposing himself into the situation (which presumably included trying Jedi mind tricks) to try to influence a group of individuals when he doesn't really know the power-dynamics at play.
“Careful, Chewie,” Han cautioned from across the square. “He must be the head honcho.”
“No,” Threepio corrected, “actually I believe he’s their Medicine Man.”
Luke was about to intervene, then decided to wait. It would be better if this serious little community came to its own conclusions about them, in its own way. The Ewoks seemed curiously grounded for a people so airborne.
Return of the Jedi: Official Novelisation
Instead he opts for a more forthright solution (but one that falls short of outright violence), by playing on their superstitious credulity.
As to the specific question of whether Ewoks can be 'tricked', the Star Wars RPG (Saga Edition) Core rulebook indicates that Ewoks are certainly smart enough to be tricked (they have an above class-3 intellect) and are insufficiently strong-willed to win a defensive roll against an experienced Jedi Master if they want to use their 'affect mind' skill on an individual Ewok.
That being said, it's also made clear that affecting 10+ individuals that you can't directly communicate with is next to impossible, so in this particular circumstance Luke might have been able to scare the Ewoks with a Force affect, but he wouldn't have been able to trick them into releasing him willingly.