No, Thrall is aware Grom was not all heroism and greatness. But he still values Grom's legacy because Grom's very last act granted the whole orcish race freedom from demonic corruption. Garrosh was leading that very people into a black hole, pretty much shitting on that legacy. He even abandoned Gorehowl down in the vaults of Pandaria to claim his new power. Rather symbolic to me.
But this aside, Thrall is not really delusional about the orcish figures of the past or his race's history, he's the very narrative voice of
Rise of the Horde after all. He simply did a terrible job in sharing his views with Garrosh, he hasn't been able to deal with that depressed dude, the son of his "big brother", and hurt his feelings. He pretty much enlightened Grom as a hero to raise Garrosh's self-esteem, until that ego grew beyond Thrall's own expectations (especially when Garrosh became Warchief).