There's really two issues being discussed here: who is a better pilot (Jazz or Seether), and how well could a Morningstar handle fighting a Lance/Dragon.
Regarding the first question, I have to give it to Jazz. Yes, Seether was genetically engineered to be a superior pilot, but that doesn't mean that he's automatically better than Jazz. Military pilots today are literally the best of the best--so many people want the job that the military can afford to take only the absolute best and leave out any that have even the slightest drawback (e.g. imperfect vision). I imagine that pilots in WC are the same way--they're the absolute best (at piloting) that humanity has to offer. Furthermore, Jazz was particularly good even as pilots go, so he's the best of the best of the best. Such "natural" selection (pun intended) should produce a person that is genetically as good as anything GE could produce. I think Jazz has the edge because of experience. Natural talent takes you only so far, and then experience is king. During WWII, pilots figured out that once someone survived their first five combat missions, they were infinitely more likely to survive their next twenty or so, because if they survived long enough to get five combat missions under their belt, their experience gave them enough of an edge that they were way better than rookies. Jazz (at the time of his death) had at least ten years of combat experience, fighting Kilrathi and humans, and often doing so in outmatched ships (Hornets versus Hriss, Sabers versus Gothri, etc.). Angel even stated that he was the best pilot on the Concordia until Blair showed up. Seether had far less experience (did he actually ever fly in the war?), and what he did have largely came flying much more advanced fighters than his opposition (Lance versus Hellcats or Banshees). So I think, all other things being equal, Jazz would have been the better pilot at the time of their respective deaths.
The Morningstar versus the Lance/Dragon is an interesting question. Obviously, the Lance is a more advanced fighter, and there's nothing that says one can't carry a Mace (although we'll assume standard loadouts here). Tactically, a Lance could win a large percentage of the time just by simply cloaking, charging up its fission cannon, decloaking on the Morningstar's tail, and smoking it with one shot (we see Seether do exactly this to Russo and Sparks in Hellcats...only Vinman manages to actually dogfight at all, if I recall correctly). However, the Mace does give the Morningstar a chance. If the Morningstar pilot can keep flying randomly enough to keep the cloaked Lance from parking on it's tail, and manage to get pointed at the Lance as it de-cloacks, its Mace can take the Lance down with one hit (as John "Starkiller" Sheridan once said, nukes don't care whether they can lock onto a target or not). I would estimate that, in the hands of equal pilots both familiar with the capabilities of both ships, the Lance would win 65-70% of the time.