Is Loki good?

By fan service you mean her giant chest beefers? Mostly yes, but she also has a good offensive statline for a healer (but nothing that you couldn't get through merging already existing 3-4* healers). Her prf staff is nothing extra tho.

I'd say she's pretty good, but don't get her if you don't care as her of a character.

This is simply because you can get a healer from the 3-4 pool.
None of those healers have the atk/spd Loki has, but they can be merged much easier and often have other good things about them.
Lucius trades speed for amazing res and good atk. You can get a perfect IV (in my opinion +spd -def) that basicly trades a bit of physical bulk for amazing res.

She can give some odd utility on potentially turn 0/1 by slowing some ranged threats (Veronica/Reinhardt/Lyn being some of the scariest threats as concerns mistakes/unpredictability), and she has a PRF staff that has synergy with a duel skill beyond the scoring element.

She's by far the best arena healer for scoring.

No, definitely a good unit if you can put up with the visual insult. Her staff's effect can help your team against too many ranged units (ex: horse emblem on tight-quartered maps); very difficult to take down in one hit due to high HP; good as a backup source of damage with wrathful built into her prf.

For more, see https://fireemblem.gamepress.gg/q-a/i-pulled-lokikliffowain-what-do-i-do