Bukele's 90+% approval rating (as measured by independent pollsters not controlled by gov't, AFAIK nobody claims those numbers to be wrong) supports the claim. If ordinary Salvadorans weren't seeing a massive improvement in their daily lives, he wouldn't be so popular. Also murder is easy to track, it's hard to hide a death, community and local news will notice. (El Salvador has independent media critical of the gov't, for example https://elfaro.net/ )