Based on that "htop" screenshot, the process that is using most CPU in your computer is the process associated with /usr/lib/mate-indicator-applet/mate-indicator-applet-complete. As I see - in that same screenshot - that, in the upper right corner of your Ubuntu MATE panel, your clock is currently configured to show the hours, minutes and seconds, I suggest that you reconfigure your Ubuntu MATE clock to stop showing the seconds (so it does not have to update the applet every second). That tip of mine worked for @mickee that had a similar problem and also had his clock set up to show the seconds. For more details, please see the following post of mine:
... and the following confirmation by @mickee that removing the seconds from the clock stopped the problem for him:
