No in that climate mid summer would not be ideal. Individuals climates would have to be taken into consideration. Earlier in the summer would probably be better. The thing with mugos is that they do better being potted and root pruned after the first flush of growth. I've seen this with the three that I have. I've repotted here in CT mid summer with temps in the 90s, but much higher humidity. I just keep them in the shade for two weeks or so and they have thrived. Maybe the timing doesn't work in Fresno. Which might explain the loathing.
There you go. I feel that when giving advice on the internet which can be read around the world, a caveat should be included "in my area" I can repot mid summer.
The loathing is not so much the fact that the timing for repotting may be crappy, but the fact that those seen in Europe and dug as yamadori with twisty wild trunks can't be had here as in Europe. Being forced to choosing material from nurseries that have never pruned the trees for bonsai use means that they have terrible whorls of branches that have caused swellings and knobs that are hard to reduce. It also means that roots have bulged up in cans left around the plants too long like many junipers suffer from.
When Mugos are grown with bonsai in mind in huge fields and left to grow for fifty years , then we may have something. Untill that time nursery crawls are the best method and finding those needles in a haystack that are far and few between.