Before anything else, you may benefit from reading my earlier post on service pokémon, as any pokémon that is kept primarily to help humans with either physical or psychological limitations through their day-to-day activities is considered to be a service pokémon. (In this sense, pokémon are a bit more flexible than animals.) This includes those pokémon that assist those who have anxiety disorders or those who simply need a pokémon to help them focus on the present. I’m throwing a link to that post in this one not to simply say “read that post” and leave it at that; that would be rather rude of me. Rather, there may possibly be information in that post that I’ve simply forgotten to include in this one.
That having been said, to answer your question, absolutely. They tend to be preferable to many other service pokémon trained to handle forms of anxiety, social or otherwise. Most Sweet Scent users have the passive ability to emit pleasant aromas with or without Sweet Scent’s assistance, and as such, they’re a bit more subtle in their approach than an empath, a Singer, or a member of the chansey line. That and this passive ability also has the added benefit of affecting all parties within its range of effect, which means the pokémon’s human companion has the added reassurance that there’s a very good chance any conversation held upwind of their partner will be pleasant.
Of course, as I’ve mentioned above, those aren’t the only pokémon that are used, and although they’re the most popular for anxiety specifically, other humans may have different preferences or needs that require the services of other pokémon. For example, some people may prefer the services of a psychic pokémon, such as ralts, to influence their emotional states. Now, keep in mind this does not mean that the psychic pokémon forcibly enters their human partner’s mind and “erases” their anxiety. That would actually be highly unethical, after all. Rather, psychic pokémon—particularly those with empathic powers (so, again, such as ralts)—have the ability to project certain auras. Think of it like feeling a rush of adrenaline when you hear a fast-paced song or feeling uneasy when you hear a low, vibrating tone. In a similar way, an empathic pokémon influences the psychic energies around itself so that it projects a certain emotion outward that your mind can sense and, subconsciously, adjust itself to. In other words, a ralts may project a feeling of extreme happiness, and your mind would pick up on it and decide for itself to feel at ease because the atmosphere simply feels safe. Thus, the process isn’t quite as invasive as it sounds. Rather, the most invasive a pokémon will ever be is to break into your mind and stop you from doing something dangerous, but obviously, those are in extreme circumstances that may not entirely be relevant to the question at hand.
Back to the point, the reason why grass-types are more preferred than psychic-types is that the passive psychic method really only works for one person. While this is fine in severe cases wherein it’s necessary to focus only on the person with anxiety specifically, it doesn’t quite have the added reassurance of guaranteeing that all human participants are essentially on the same emotional plane.
The same, of course, could be said for the less subtle pokémon, such as those with Heal Pulse (which can be soothing for some people), those who can emit soothing sounds (such as the chimecho and jigglypuff lines), and so forth, although these are seen as less desirable options than either Sweet Scent users or psychic pokémon. While some people find healer or auditory pokémon to be the most effective for their needs because they (the humans) simply respond better to healing waves or soothing melodies, these pokémon may also call attention to one’s social anxiety, which some feel defeats the purpose of having a service pokémon specifically to help them in social situations.
In short, there are a wide variety of different pokémon that may suit your needs. It depends on the specific effect you’re looking for and what your needs specifically are. After all, while it’s true that social anxiety is something specific, there are many different ways to approach it, depending on how severe it is and what a person’s situation may be.