Firstly, it is imoprtant to understand that mons that are good in normal gameplay doesn't necessarily mean that it is good in VGC. I can helper answer your second question because I play VGC a lot: (Assuming you are talking 2v2)
There are a few points for this:
1) Inteleon faces bad competition.
Although a first glance at Inteleon's 125 sp.atk and 120 speed may seem impressive, it is important to understand that it also faces competition from Primarina, Urshifu, Kingdra, Kingler, Dracovish and Lapras. Inteleon's niche sp.Atk and speed stat may look nice on paper. However, the moment it enters the battlefield, it faces stiff problems:
252 SpA Inteleon Ice Beam vs. 0 HP / 0 SpD Rillaboom: 134-158 (76.5 - 90.2%) -- guaranteed 2HKO after Grassy Terrain recovery ------ Assuming that this is not Series 6, Inteleon will have trouble OHKOing Rillaboom.
252+ Atk Choice Band Rillaboom Grassy Glide vs. 0 HP / 0 Def Inteleon in Grassy Terrain: 354-416 (244.1 - 286.8%) -- guaranteed OHKO ------Grassy glide is more than enough to take out Inteleon, even without Choice Band.
But maybe that's a bit too drastic. I'll give another example:
252 Atk Life Orb Dragapult Dragon Darts (2 hits) vs. 0 HP / 0 Def Inteleon: 150-180 (103.4 - 124.1%) -- guaranteed OHKO
Inteleon's crippling defenses make it extremely vulnerable to even not-super-effective attacks, as portrayed with Dragapult...
In more recent cases, Inteleon has seen increase in usage in series 6. However, the ubiquitinous of Amoongus confines it. More than ever, players are just switching Amoongus into fairy and water types and spamming rage powder, rendering Inteleon useless:
252 SpA Sniper Inteleon Snipe Shot vs. 252 HP / 72+ SpD Amoonguss on a critical hit: 76-90 (34.3 - 40.7%) -- guaranteed 3HKO
Amoongus also means Urshifu. And a lot of Urshifus carry thunder punches to counter Primarina, which will be more than enough to take out Inteleon.
However, I do believe Inteleon is underrated too. Snipe-shot sets alongside with scope lensn might not be the best option. However, I believe focus sash or choice specs Inteleon poses a huge threat, becoming a fearless revenge killer with its diverse movepool. However, it is also because of the rising usage of other water mons such as Primarina, Kingdra, Lapras, Urshifu and Dracovish that cripples Inteleon's usage.
Primarina has dual typings, which allows it to fill that STAB Fairy coverage in a team, alongside with STAB Hyper voice to deal massive damage to both Pokemon.
Kingdra has become increasingly popular, due to its Gmax-Move, allowing it to lower the speed of opposing Pokemon, which cripples Inteleon.
Urshifu has dual typings and can become a huge physical-late-game-cleaner or a lead, both of which is hugely menacing due to its massive 130 BP attack.
Dracovish has gained in popularity in Series 6, with Scarf+Fishious Rend dishing damage to that of a Z-move.
However, Inteleon only possesses the capability to revenge kill and will most likely be an OHKO mon. Inteleon lacks the capability to also wipe-out neutral effectiveness mons such as the following:
-Dusclops
-Urshifu
-Ferrethorn
-Clefairy
-Lapras (who can freeze-dry)
-Grimmsnarl
-Dracozolt
-Gastradon (who will love to switch in)
-Porygon-Z
-Sylveon
Its single-typing is another one of its harsh drawbacks, not being able to deal STAB ice beam or dark pulse.
Inteleon is a threat. However, it is not an overwhelming one in 2020 VGC, unlike its rivals and competition it faces.
EDIT: There are also a lot more mons who are capable of revenge killing much better than Inteleon too:
Gengar, with higher sp.ATK stat and can suicide-bomb with destiny bond.
Volcorona, with much higher sp.ATK and can dish out gnarly damage, regardless of super-effective or not.
Jolteon, who takes a lot of water types by surprise, possessing much higher speed and ability to at-least tank 1 or 2 special hits.
Rotom Wash, who is so diverse with its usage, being able to use nasty plot or scarf + trick or just be annoying with volt switch. It is also much more tankier than Inteleon and can LIVE super-effective hits such as Solar Beam from Venasaur with the correct investments.
