I'd say May's far more responsible than the right (if you mean ERG). She chose to court them for party management reasons, rather than the good of the country. She'll be justifiably remembered as the worst post-war PM.
I still reckon Cameron will probably go down as the worst overall. Those who already disapproved of his economics will view him as a fairly destructive figure who created the conditions for Brexit in the first place by driving people into poverty through needless austerity, and any arguments he did it for the good of the economy have basically been covered by an ocean of piss in the last few years.
May's undoubtedly horrendous, and made the mistake of basically going full-on, Lee Cattermole hard Brexit the moment she stepped into office when she knew it was never achievable, but she was probably always fecked irrespective of what path she took, even if her stubbornness has damned her to a far greater extent than was necessary.
Overall I can see her being remembered as a weird oddity, someone whose entire premiership was dominated by an issue over which she had little control, maybe a bit like Eden in that regard - albeit without the gravitas he'd perhaps built up beforehand.
I think there's a solid argument that while she was viewed as one of the government's more competent and experienced main figures before taking over, that her spell as Home Secretary was worse than her time as PM. Someone who spent years scapegoating migrants and blaming them for the country's ills while failing to even approach her targets to reduce immigration, and also someone who largely established the political conditions which preceded Windrush.