[Ketho]

Checking for a boolean true is unnecessary in most cases:

if MBFlaresDB.partyShow == true then
if MBFlaresDB.partyShow then

You should use the [ CODE ] tag or [ highlight=Lua ][ /highlight ] to retain the indenting:

function MBFlares_partyChecker() -- hide/show in raid
    if MBFlaresDB.partyShow then 
        if GetNumRaidMembers() == 0 then -- NOT IN RAID
            if GetNumPartyMembers() == 0 then -- AND NOT IN A PARTY 
                MBFlares_mainFrame:Hide()
            else -- IN A PARTY  
                MBFlares_mainFrame:Hide()
            end
        else -- IN A RAID
            if IsRaidLeader() or IsRaidOfficer() then -- ABLE TO MARK
                MBFlares_mainFrame:Show()
            else -- BUT NOT ABLE TO MARK
                MBFlares_mainFrame:Hide()
            end
        end
    else
        MBFlares_mainFrame:Show()
    end
end

You could also just check against the Instance Type, since the player needs to be inside the raid/instance to mark the raid target mobs
(only "downside" is that this method doesn't show it outside the party/raid instance)

function MBFlares_partyChecker() -- hide/show in raid
    if MBFlaresDB.partyShow then
        local _, instanceType = IsInInstance()
        if instanceType == "party" then -- IN A PARTY
            MBFlares_mainFrame:Show()
        elseif instanceType == "raid" and ( IsRaidLeader() or IsRaidOfficer() ) then -- IN A RAID AND ABLE TO MARK
            MBFlares_mainFrame:Show()
        else -- NOT ABLE TO MARK
            MBFlares_mainFrame:Hide()
        end
    end
end