did anyone else think upon first reading poetaster that it seems like a combination of poet and disaster?

No, but then I've studied Latin. The pejorative suffix -aster occurs in a number of Latin words: e.g., claudaster 'slightly lame', oleaster 'wild olive', philosophaster 'bad philosopher', pueraster 'stout lad'. OTOH, disaster is a combination of the pejorative prefix dis- and the noun aster 'star'. It means literally 'bad star' in Latin. A more common misanalysis of the word would be poe taster, whatever that might mean.

Ceci n'est pas un seing.