Suppose you had 80% up quarks, and only 20% down quarks. How would this affect stellar formation?
It's really not clear what hypothetical limits you're imposing. I take your question to mean that in the process of baryogenesis the various baryons like protons and neutrons highly favored up quarks (lots more protons than neutrons). Remember, quarks are subject to confinement so other than a quark-gluon plasma, quarks are confined to baryons.
Since stellar formation depends on hydrogen and hydrogen formation depends on protons and electrons primarily, it's unlikely that there would be any meaningful impact.
I'm pretty sure even if there were some wild physical law that forced there to always be a huge imbalance of up to down quarks there would still be enough hydrogen formation for stars to form. It's not clear what this restriction would do to hydrogen to helium fusion though.