Is an MLA citation needed for a broad description of a poem's theme?
In general, it's always a good idea to cite your sources. This can especially be the case with older poetry, where there are often differences in spelling, punctuation, and layout between different editions of the same poem. 

So, at the first mention of a literary work, whether it's a direct quotation or an indirect reference, it's probably a good idea to cite the specific version of the poem you're writing about. This will make life easier for everybody involved. You would cite the overall poem, usually as a publication in a larger source, and follow the specific guidelines of whatever style manual the publication you are writing for (or the school you are attending) is using.

For example: 


  Poetovsky, R. "Sonnet 100" in Every Sonnet Ever Written by Anybody, ed. John Smith. Random Publisher, Some City (2000).


Then, if you go on to cite the poem again, you can refer to the specific lines as needed; general statements probably wouldn't need to be cited.