I think this is a very low level of offence. As Keith says, its the kind of thing people say about other people anywhere - could also apply to a visitor to Scandinavia, for instance.
There is certainly a common and very mild racist attitude to Thais and people who marry them - but not something that has ever worried me or particularly offended either of my ex wives. And its nothing compared to the racist attitudes that trouble people from other races in the UK.
Ian