It depends.
DLR has a number of things different from WDW and one of them is the impact of annual pass holders. For more on this see this old thread I wrote: "Understanding AP Blockouts and Impact on Crowds"
www.disboards.com/showthread.php?t=2365983. In particular there is a trend related to the "SoCal" AP holders (SCAPs).
These have a summer blockout which begins for good on June 29. See this link and screen shot below
https://disneyland.disney.go.com/passes/blockout-dates/#
View attachment 292531
What happens is that DLR opens new things for the summer and SCAPs try to squeeze in final visits before the blockcout. That causes some extra crowds.
I myself am a Deluxe AP holder (DAP, not SCAP) and am going June 19-22 to avoid the SCAPs (except for June 19, an arrival day for us with half day in the parks) and be there after the Grad nights have concluded. See "The Scoop on Grad Nights at DLR in May and June"
www.disboards.com/showthread.php?t=2298400
So June can be good or not so good for crowds. Depends on the exact day. One of my co-workers went in 2017 on the equivalent of Tuesday June 19 and Friday June 22 for just those two days. She said it was a night and day difference on crowds.
The average temperature does increase as the summer goes on. The averages are actually not too bad and not nearly as bad as WDW in the summer. The challenge is when you are there on an above average temperature day. "Detailed Weather Data for DLR"
www.disboards.com/showthread.php?p=23004158.
I myself have gone multiple times in June, July and August and would not hesitate to go in any of those months. If I could choose my ideal time it is late June and it becomes less ideal as the summer goes on.
For the most part it is easiest to manage hot temperatures when you make a good hotel choice (nearby, preferably walking distance IMO), get to the parks early each day and take afternoon breaks. See "HydroGuy's Hotel Rule"
www.disboards.com/showthread.php?t=1199862