In 5e, specifically, the DM narrates the results, of player actions, yes.
Since 3e, D&D has prettymuch let players describe things about their characters as they like. So even if there were a visual image, it wouldn't be unfair to substitute another. Conversely, if there were a specific image, it would doubtless also be used as grounds to deny the use of the feat.
Sentinel says "You have mastered techniques to take advantage of every drop in any enemy's guard" - so, I suppose that covers just about anything you might do. The Kaiju steps over your character, leaving vulnerable spot exposed, you stab it, it rears back in pain and surprise - damage, 0 movement for the rest of the turn. Doesn't seem hard to come up with.
Really, the only thing implausible about all that is the Kaiju ever having it's guard up against such a relatively small foe, in the first place. It'd've been as reasonble to have ruled her able to use Sentinel every round she stayed adjacent to the Kaiju, and just driven it crazy until it finally took the trouble to stomp her flat.