With regard to Shelley's "Feelings of a Republican on the Fall of Bonaparte," I believe that the sentiment the author expresses is anything but cordial.
Shelley's first line quite simply states, "I hated thee, fallen tyrant." In my opinion, I do not believe there is sarcasm used in this poem based upon the thread of disgust the author seems to have for the "fallen" Bonaparte. The tone is extremely malevolent as the author carefully uses vivid and extremely negative words and phrases like:
The theme of the poem seems to point to man's disregard for justice and "virtue" in the world by putting himself and his interests, misplaced, before all others. The theme is expressed early in the poem, when the poet accused Bonaparte of joyfully ("dancing") destroying "Liberty." The use of "unambitious" seems to indicate that Shelley believed that Napoleon did not have an inspired plan to change the world—to make it a better place. Labeling him a "slave" would indicate how low the author feels about the man and his standing within civilized society.
I did groan
To think that a most unambitious slave,
Like thou, should dance and revel on the grave
Shelley tells the dead tyrant that he could have made a difference and established a new kind of world for the French, that would live on:
Thou mightst have built thy throne
Where it had stood even now...
However, Shelley goes on to accuse Napoleon of taking a road which followed no sense of moral integrity, choosing to raise himself at the cost of lives, the spilling of blood, which "Time" has destroyed, indicating that all the French revolutionary did was for his own vanity ("pomp"), and is worthless—amounting to nothing of value: gone.
...thou didst prefer
A frail and bloody pomp, which Time has swept
In fragments towards oblivion.
Shelley admits that he prayed that "massacre" would overtake Napoleon and all that he stood for: treason and slavery, rapine (seizing goods from others), fear and lust, and destroyed he who was the "minister" or leader of all these hated elements.
For this, I prayed, would on thy sleep have crept,
Treason and Slavery, Rapine, Fear, and Lust,
And stifled thee their minister.
Shelley admits that the knowledge comes to him late, in that Napoleon and France are destroyed, that "Virtue" (which he personifies here as having a "foe") has an enemy that is more eternal than the mortal Napoleon, or the military force or deceit that he visited upon the world. Virtue's foe lies in sticking to the old ways instead of changing for the better, committing crimes that are passed off as "legal," and "faith" that comes at the price of bloodshed. The line "foulest birth of Time" makes me think that Shelley is saying that Time will forever produce people like Napoleon as long as the world lasts.
Too late, since thou and France are in the dust,
That Virtue owns a more eternal foe
Than Force or Fraud: old Custom, legal Crime,
And bloody Faith, and foulest birth of Time.
Shelley seems clear in his hatred of Napoleon—how he lived and treated those who could not defy him. His theme seems to concentrate on what Napoleon could have done for the good of all, compared to what he chose to do, which served no one but himself.
Please note that poetry speaks to different people in diverse ways. These are my impressions.