Grammar, or the literal tranlation is not helpful without knowing the context in this case, but either way, here it is:
If only Hamburg was CDU's biggest problem
It is technically indeed Konjunktiv II (ist → wäre). But it doesn't express wishful thinking, imagination, or courtesy.
Technically, this headline could mean many things, depending on context. It could for example mean the author is aware of bigger problems but wishes that the CDU indeed did not have these, and that the poor outcome at the election (not surprising in any way, to be honest) was something they could just ignore.
In reality, the author, being an author of a known left-aligned paper, is being spiteful. It's scoff, not compassion, nor wishing. The author wishes to further inflate other existent or non-existent problems (which he does in a far from impartial way, too, in the article).
Although the article is called "an analysis" and tries to mask itself as that, it is of course far from being that. Without knowing where the line came from, it is however hard, if not impossible, to know.
Politics are probably a bad thing to read for practicing German at this time because it is troublesome to find something that isn't misleading or outright lies.