Would we be happier if there were no mega corporations, only normal sized businesses?

yes

I've got a feeling that mega corps aren't really efficient at using resources or generating net positives for society. They are really good at creating tiny pockets of obscene amounts of wealth and power though

Yes because wealth would be better distributed and we could keep more money within our communities by supporting more local businesses.

Those mega companies have enough money to spend on experiments that lead to improve human conditions