Can anyone tell me if this is a good thesis statement, and if not, what I can do to improve it?"Known as a metaphysical poet, John Donne’s love...

That is an interesting thesis statement.  I would just change a word or two so that it read like this: "Known as a metaphysical poet, John Donne’s love poems convey both his true life experiences as well as humanity's struggles in everyday life." That is, unless you are trying to say that the "struggles in everyday life" that you are talking about are those specific to Donne.  If that is the case, I would consider wording it like this: "Known as a metaphysical poet, John Donne’s love poems convey his true life experiences, including his struggles during everyday life." Personally, I prefer the first version.

I am not sure that the first bit matches the last, though.  "Metaphysical" usually has something to do with trying to explain the nature of reality or discussing something that is beyond the physical laws of reality.  What your thesis seems to be saying is that "because Donne is known as a metaphysical poet his love poems have his true experiences and the struggles of people's everyday lives inside them."  To me, they just don't match.  It might be more apt to say "Even though Donne is known as a metaphysical poet, his love poems contain images that come from both his own experiences and the observations he makes regarding the struggles of other people."  This better explains, from my standpoint, why a poet known for thinking "metaphysically" would use such concrete examples.

That's just my thinking on it, though.

I like your outline, but I am not sure about the A,B, and C part.  They just don't seem to match your thesis, but I suppose without seeing them written out it is hard to tell.  Everything in your introduction should support your thesis statement, which says nothing about the conventions of poetry named in points A,B,C.  Does that make sense?

As for your "love poem vs. Sonnet" conundrum, well, that would depend, but I don't think it matters a whole lot. A sonnet is a poem, but a poem is not always a sonnet.  You'd have more of a problem if you called them "sonnets" (which implies the structure of a sonnet) and they weren't.  So that's the question...are all the poems you are talking about written in sonnet structure?

Anyway, hope this helped.  Good luck to you.