Only if you think it’s interpretable and you know what values of RMSE are good or bad. Some people find MAE easier, and in one case I worked the application experts preferred 90% quantile of absolute error (ie, in 90% of cases the error is smaller than the given value).
preds has draws from the posterior predictive distribution and
log_lik is the same as is used for elpd, then you can compute LOO RMSE as
# LOO predictive means
# LOO RMSE