Having lived in both places, I'd say America is worse. Slavery, lynchings, police murders based on race, institutionalized apartheid like Jim Crow, needing movements like BLM, all this puts America on the level of South Africa back in the day. Severe institutionalized racism.

Thailand is more about discrimination. None of those things I mentioned about the States exists anywhere near the same scale and degree in Thailand. Discrimination is absolutely rampant but generally low-key in the day to day. It goes without saying that Thais are "better" than all non-Thais, (except maybe for Japanese who they seem to admire for their clean orderly society) but these are typically opinions and attitudes, not overt actions for the most part. Dark skin is lesser than light skin aesthetically, but no one is going to go "do something" to dark skinned people just because they're dark.