These big shiny green beetles are well known as fruit eaters. When we had the fig tree in the nursery, they lived up to their common name 'Fig eater beetles' by attacking and eating the ripe figs enmasse. Actually, that's one reason we got rid of that fig tree. The other being, don't particularly like figs or stepping in the fallen ones while doing chores.
But I've also seen these beetles spending a lot of time on flowers, like this one on the bulbine frutescens. Since Cotinis mutabilis is attracted to sweets (fruit, fruit juice) they could be eating nectar and maybe, incidentally, flower parts. Cotinis are in the larger group of fruit and flower chafers (Cetoniinae) which includes many flower eating beetles.
It's almost the end of Green Peach beetle season and soon we will miss their big clumsy noisy flights over our heads and bumping into things. The eggs are laid in the mulch or manure, the larvae will grow over the winter and emerge next summer. I guess if you grow fruit you would consider these a pest, but we see them as a sign of summer and a cause for wonder:
If a creature this shiny and clumsy and loud can thrive, Earth must be a friendly place.
Ha. Actually all they require is a likely pile of mulch or compost, and a lack of flooding rains over the winter and the next generation is almost assuredly going to emerge. They fly far (ever been in the middle of a large parking lot and been buzzed by one?) so if you're trying to eliminate them from your fruit trees, you'll need to get your neighbors to participate in mulch removal as well.