To start, this isn't a word you want to say out loud as you look at your computer in the middle of a cafe in Japan or something. I guess you could say that Japan doesn't really have any swear words, just rude ways of saying words, but this is a rougher word, so be careful when using it, if you ever do. This came about because I asked my host father the other day what ド meant; he basically told me it's something I probably would never use. I saw it used on a television program, and while I recognized the word that came after it, I was unsure really what ド added. Apparently, it's to add emphasis in a manly way. You can only add it to certain words, but I'm not sure what the criteria are.
I've never been called へたくそ (not to my face, at least), but I have heard it. It happens to be one of the words you can add ド to. Lousy/unskilled/poor・下手・へた is an easily recognizable word - it's just the opposite of 上手・じょうず. If you want to say "really bad at", you can say へたくそ、くそ being equivalent to 'crap' (and other similar words). Now you are free to use this word to criticize rudely criticize music/musicians you don't like. Do you have really terrible soccer skills? You might be called へたくそ。
So if 下手 means you're poor at X, へたくそ might be comparable to you're really terrible at/ you suck at X, and どへたくそ could mean you have no f-ing chance at ever being good at X.
Here's a bit more about how you can use it. Japanese-reading practice!