There is no such function in THn, but you can easily compute it, looping at the bins. Here is an example code, assuming the histogram is of double content type:
int n = hist->GetNbins();
auto & array = static_cast<TNDArrayT<double>&>(hist->GetArray())
std::vector<double> x(n);
std::generate(x.begin(), x.end(), [ibin=0]() mutable{ ibin++; return array.At(ibin);});
double maxValue = *(std::max_element(x.begin(),x.end());