The 2020 film of it was worse. I didn't even realise until 90% of the way through that Robert Downey Jr was even trying to do a Welsh accent, and spent the rest of the film wincing. Dick van Dyke, all is forgiven...
I've never seen the 2020 film, but I do know that the Welsh accent is notoriously difficult for anyone even from the United Kingdom to impersonate, at best they come of sounding Geordie from the north-east of England. An American would certainly struggle to do a Welsh accent. Australian and New Zealand accents are also extremely difficult for overseas people to impersonate, and while its very rare for overseas actors to try a New Zealand accent, many have tried and failed to do an Australian accent. One of the few to do it successfully is American comedic actress Kate McKinnon.