[myrroddin]

You can go even simpler. On a side note, I added local to the first line.

local HiddenFrames = LibStub("AceAddon-3.0"):NewAddon("HiddenFrames", "AceEvent-3.0")
 
function: HiddenFrames:OnInitialize()
    -- called when the addon is loaded
end
 
function HiddenFrames:OnEnable()
    PlayerFrame:SetAlpha(1)
    TargetFrame:SetAlpha(1)
    
    self:RegisterEvent("UNIT_ENTERED_VEHICLE", "CheckVehicle")
    self:RegisterEvent("UNIT_EXITED_VEHICLE", "CheckVehicle")
end
 
function HiddenFrames:OnDisable()
    PlayerFrame:SetAlpha(1)
    TargetFrame:SetAlpha(1)
end
 
function HiddenFrames:CheckVehicle()
    if UnitInVehicle("player") then
        PlayerFrame:SetAlpha(0)
        TargetFrame:SetAlpha(0)
    else
        PlayerFrame:SetAlpha(1)
        TargetFrame:SetAlpha(1)
    end
end