He got a bad rep, mainly back around the period where he was a starter, because bad Loki players are heavier than almost anything else since he's one of the squishiest frames around and his stealth requires more care than the bulk of "press to win" abilities. Hence jokes about "Loki mains" even though the bulk of the playerbase weren't around when that was a thing.
FWIW I think I can fairly call myself a Loki main, given that ~20% of my playtime is on Loki and ~25% is on Loki Prime. The real reason why Loki/Loki P is great is because he is the best-balanced frame in the game. This is attested to by his changelog--more specifically, how empty it is. He was one of the eight frames present in Closed Beta. Over the ~6ish years since CBT, he has been present in maybe five or six changelogs. Those were all sound/visual/ability scripting bugs. To my knowledge there has never been a balance change made to Loki, other than the addition of Radial Disarm when warframes moved from three to four abilities, and the patching of his CBT ability to Switch Teleport with defense objectives.
Loki has never been buffed or nerfed directly. He is the quintessence of balance as far as warframes are concerned.
Decoy is occasionally useful as a distraction, and as the 1-3 combo to circumvent Spy barriers, AoE zones, and (before the parkour rework) wide gaps. Not great, but most frames have an ability which is so utterly worthless it will only ever be used if someone accidentally fat-fingers the key. Loki's worst ability still has marginal utility.
Invisibility is, as I suggested before, the perfect damage avoidance ability from a balance standpoint, because it relies on the player continuing to pay attention and play smart. If you fuck up while you're invisible by walking into an AoE/nullifier/skaterboi/near a teammate who is busily drawing fire, you get punished. If you don't watch your timer, you get punished. It gives you a lot of latitude to pull off sneaky stuff, but still imposes limitations which help keep gameplay engaging that frames like Ivara, Rhino, and Limbo lack.
Switch Teleport is useful for bypassing Spy obstacles in conjunction with Decoy, removing Ancient Healers from blobs of enemies, teleporting dangerous enemies off cliffs, getting idiot downed teammates out of dangerous places so they can be revived, rapid movement even with Movement 2.0, moving escort objectives around (including eidolon lures), &c. It's not spectacular, but it's a solid multi-purpose utility.
Radial Disarm, with or without the augment, is one of the most effective mass CC abilities in the game, and scales infinitely as it ignores enemy level, armor, &c. It also lacks the damage necessary to serve as a cheese nuke for focus/affinity farming. If you do use ID, he doubles as a Nyx without the chance of random Bombard rockets gibbing you.
That's Loki in a nutshell. He's not the best at anything, but he's good at almost everything. He can break a lot of content, but never bereft of risk and only if you keep playing carefully. That's why he has never been nerfed, and why he hasn't been buffed despite being overshadowed by a lot of power creep (which often has been nerfed despite being more recent).