I have a Mid-2012 MacBook Air with a core i5 8 gigs of ram with macOS Mojave, and have no issues. sure, I cant really hungry task, but lets be real, its a dual core processor from 2012. anyhow, usually my computer sits at 60 to 65 degrees celsius when doing basic tasks like web surfing, video streaming, and going through mails. when I start using photoshop on my second monitor (a 1080p monitor) the processor kicks up to 85 until I boost the fan speed to 100% at which point it goes back down to 65-70 degrees. what I'd recommend would be to restart the computer if it hasnt already been done, install "iStat menu" and buy a key to use it ( about 15$, usable on 5 apple devices) to have a clearer idea of what is happening, and maybe check on activity monitor ?
Edit : had this computer for about a year and a half now, had High Sierra before, and was having the same temperatures as I have now, only have the processor being used a tad bit more while idle ( at about 10% usage while at 5-6% before)