This should do the trick:
<<if $dex > 99>>
<<set $dexTier = 5>>
<<set $dexDesc = "Lightspeed">>
<<elseif $dex <= 0>>
<<set $dexTier = 0>>
<<set $dexDesc = "Standing Still">>
<<else>>
<<set $dexTier = Math.floor(($dex - 1) / 25) + 1>>
<<set $dexDesc = ["Sloth", "Kinda Slow", "Pretty Fast", "Supersonic"][$dexTier - 1]>>
<</if>>
The Math.floor() method removes anything after the decimal place, so that should turn any numbers from 1 to 99 into 1 through 4. Then it uses that tier to pull the correct description string from the array.
That also fixes the problems in your code above where you weren't testing for $dex == 1, where you were setting $dexTier instead of $dexDesc, and corrects "Super Sonic" to "Supersonic".
Hope that helps! :-)