Yes, most definitely all improvised weapons can be ranged. To understand why I say that, read on:
First as Jeremy Crawford points out: http://www.sageadvice.eu/2015/11/12/magic-stone-sneak-attack/
Attacking magic stone with a sling is just a spell attack with a sling. (Corollary, using it by hand is just a spell attack with an improvised weapon).
First the rules of sneak attack:
Beginning at 1st level, you know how to strike subtly and exploit a
foe’s distraction. Once per turn, you can deal an extra 1d6 damage to
one creature you hit with an attack if you have advantage on the
attack roll. The attack must use a finesse or a ranged weapon. You
don’t need advantage on the attack roll if another enemy of the target
is within 5 feet of it, that enemy isn’t incapacitated, and you don’t
have disadvantage on the attack roll. The amount of the extra damage
increases as you gain levels in this class, as shown in the Sneak
Attack column of the Rogue table.
Now, the rules on improvised weapons:
If a character uses a ranged weapon to make a melee attack, or throws
a melee weapon that does not have the thrown property, it also deals
1d4 damage. An improvised thrown weapon has a normal range of 20 feet
and a long range of 60 feet.
Now lets examine the thrown property. (True the wording of previous doesn't technically grant 'thrown' but for additional support.
Thrown. If a weapon has the thrown property, you can throw the weapon
to make a ranged attack.
Next lets look at the definition of ranged weapon within the PHB
Every weapon is classified as either melee or ranged. A melee weapon
is used to attack a target within 5 feet of you, whereas a ranged
weapon is used to attack a target at a distance.
Since the stone is improvised it gains an attack range of 20/60 per the rules of improvised. Per the rules on what determines what a ranged weapon is "is used to attack a target at a distance" the pebble is a ranged weapon.
Additionally, lets examine the definition of "Range".
Range. A weapon that can be used to make a ranged attack has a range
in parentheses after the ammunition or thrown property. The range
lists two numbers. The first is the weapon's normal range in feet, and
the second indicates the weapon's long range. When attacking a target
beyond normal range, you have disadvantage on the attack roll. You
can't attack a target beyond the weapon's long range.
Since the rules of improvised weapons provide the weapon a range increment, it confirms that the stone without a sling is still considered a ranged weapon.
Put very specifically, by RAW anything can be turned into a 'ranged' weapon. Even a dead cat. Applying magic stone to the pebble doesn't change the fact that it is already considered a ranged weapon. The only thing that changes is that it becomes a spell attack with a ranged weapon versus a standard ranged attack with ranged weapon.
On that note, lets examine the Booming Blade cantrip.
As part of the action used to cast this spell, you must make a melee
attack with a weapon against one creature within the spell’s range,
otherwise the spell fails. On a hit, the target suffers the attack’s
normal effects, and it becomes sheathed in booming energy until the
start of your next turn. If the target willingly moves before then, it
immediately takes 1d8 thunder damage, and the spell ends.
This is a spell that requires a melee attack roll. Could someone using this spell not benefit from sneak attack on the original attack roll just because they are utilizing a spell? The answer is no, of course they could use sneak attack assuming the weapon was a finesse weapon or had advantage. The fact that they utilized a weapon, and made an attack roll to utilize the spell effect are all that's important.
The same can also be said for steel wind strike (yet your capped at one target of sneak attack per turn, require advantage and finesse). I.E. It's a melee attack that uses a spell attack roll instead.
You flourish the weapon used in the casting and then vanish to strike
like the wind. Choose up to five creatures you can see within range.
Make a melee spell attack against each target. On a hit, a target
takes 6d10 force damage.
You can then teleport to an unoccupied space you can see within 5 feet
of one of the targets you hit or missed.