You can still cast the spells
Yes, the Feral Tiefling still uses Charisma for their Spellcasting trait.
The "Feral" variant of the Tiefling is one of several variants that can be used to modify the standard racial features, from Sword Coast Adventurer's Guide. The lore behind these differences is given on page 118, and allows you to combine several variant features
Tieflings in Faerûn generally have the racial traits of tieflings in the Player's Handbook, except that those not descended from Asmodeus might exhibit different qualities; see the "Tiefling Variants" sidebar.
The Sidebar lists the variants Appearance, Feral, Devil's Tongue, Hellfire, and Winged, the last three of which replace or modify your Infernal Legacy feature (and are mutually exclusive).
Spellcasting still works with your normal Charisma
Firstly, a Tiefling with Infernal Legacy (or one of its replacements with Spellcasting such as Devil's Tongue) can still cast spells using Charisma, they merely may have a spell DC or attack that is one point lower because of the lacking increase on their Charisma score. That still is a cool feature, even if your character concept is a Tiefling Rogue that needs Dexterity a lot more than Charisma and therefore opts for a Feral Tiefling. Unless your prime ability score for your class is going to be Charisma, Charisma-based Spellcasting is going to be a nice side-ability, not your main way to be effective, so the slighly lower bonus will not matter that much.
Some variants do not have Spellcasting
Secondly, there are variants that can replace the Infernal Legacy trait which is providing you with Charisma-based spellcasting, for example
Winged. You have bat-like wings sprouting from your shoulder blades. You have a flying speed of 30 feet. This trait replaces the Infernal Legacy trait.
So one answer to your question is that for a winged feral Tiefling, there is nothing lost by not having higher Charisma, as it has no Spellcasting feature that requires Charisma, and mechanically may benefit more from better AC and ranged attack that Dexterity provides.
It's OK to not be fully optimized
Lastly, character building is not all about optimization. You can have as much fun with non-optimized builds as long as they realize your character concept, and your overall playgroup is playing like that.