I don't think one exists. You can turn it on, but it requires being able to edit the scripts of the game. Also, turning on debug mode that way currently causes an issue where if you soft reset the game it will corrupt your data file and you have to unpack it again. I wouldn't really recommend it unless you specifically want to set up specific runs or similar. And even then its probably easier to ask someone to trade you an alternative starter or whatever.
The big point of IVs aren't final stats, since outside of an outright competitive environment like Showdown (where you can just set all of this anyway), the only real use for IVs is your Hidden Power typing. Which means that for a physical attacker they're basically irrelevant. And hidden power in Rejuvenation just cares if the value is odd or even, not what it is (this is untrue is Pokemon Reborn which uses the old formula). So long as you aren't just actively using say a physical attacker with 0 Attack and Speed IVs there's pretty much no difference. I tend to aim for somewhere over 15 in every relevant stat for Pokemon I'm not going to use hidden power on. Going higher will technically give you a more powerful Pokemon, but the difference is miniscule. Because of that, the actual play difference between a party of absolutely perfect Pokemon and one you are likely to catch (again, so long as you don't just go with the worst IVs you can find) isn't really significant.
My general point is that making IVs always be 31 wouldn't really change much, but would have the downside of making hidden power pretty much completely useless outside of a Pokemon that really wants Hidden Power Fairy. Or the entire Hidden Power mechanic would need to be overhauled.