Your repair is OK, however, sharkbite does make a
Push To Connect fittings for PEX. No clamps needed
I'm assuming it is a shower only, no tub. The hot or cold side? One fitting slightly smaller should not make that big of a difference. All new tub/shower faucets have a anti scalding spool built into the cartridge. It will close off the hot side some if the cold water side pressure drops significantly by a toilet flush, a washing machine filling. etc. Sometimes these spools get jammed. All I can suggest is turn off the water again and open the faucet somewhere in the middle. Tap the shower handle with a plastic hammer or screwdriver handle to see if the spool can't be jarred a bit. Turn the water back on leaving the faucet open and check again. If no change you may have to remove the shower faucet cartridge. All I can suggest here from Florida.