First the good: This poem has a light and fun tone that makes it pleasurable to read. The semi-fantastic setting was pretty fun as well and I would have preferred seeing the mouse slay the dragon using a clever way or method, but this'll do.
Now, the bad: For a story poem, the ending seemed rushed and predictable. There's also a slight leap in logic on the "finding the ring in the pocket" and "mouse slaying a dragon" bits, but as this is a fantasy story, those are easily let go.
For a poem, there is a rhyme, but the meter seems to be off, which distracts a poetry reader. Rhymes are fun and all, but having an irregular meter makes it seem off in beat. The ending two sentences seem to distract as well since the rhyme vanished all of the sudden.
Also, "was slew" should probably be "was slain".