If you're making poffins by yourself, without linking up with any other players, those can't help but have very poor performance. When playing solo, you only get to put in a single berry per poffin, and no matter what berry you put in, the sheen will be greater than its level.
The idea is that each time you feed a poffin to a Pokemon, you add its values in the relevant contest stats, plus the sheen, to what it already has. If the sheen is already 255, you can't feed any more poffins, so you're stuck with the stats you already have, and if the beauty has not yet reached 170 (⅔ of the way full) at that point, and you've already saved the game anyway, that particular Feebas will never be able to evolve unless you transfer it forward to a game where the Prism Scale method can be used. You'll have to start over with a new Feebas that has empty contest stats, and try again.
If you must try to make single-player poffins, the type of berry that provides the single best beauty stat as a percentage of sheen is the Apicot Berry, where a cooking session that isn't sped up at all but goes without any mistakes will be worth 28 beauty, 18 tough, and 39 sheen, so beauty will be 71% of sheen and after feeding all 7 poffins you can before you hit the feed cap, that will be enough beauty. In fact, 6 poffins may be enough if you get some slight improvements to cook time. However, the only ways to get an Apicot Berry are by earning it in one of the GameCube games (Colosseum or XD), attaching it to a Pokemon, sending it back to the GBA game, then transferring it forward through Pal Park where it still holds the item...or by connecting with the Wii game Pokemon Battle Revolution. Thus, you might not have access to it.
Otherwise, the next-best option is the Wiki Berry, which cooks into a poffin with 24 sheen, and provides 14 beauty (more if you cook faster, less if you let it burn) and no other stats. However, a Pokemon can only eat 11 of those poffins before it runs into the feed limit, and with no speed bonuses that's only worth 154 points in beauty, which is not enough. You can do slightly better by taking advantage of how the feed limit works: instead of 11 Wiki Poffins, make only 9 of them, and aim to get at least some of them fast enough that they bump up to level 15. After this, you'll have about 130 points in Beauty, and exactly 216 sheen. Next, run around Amity Square, talking to your follower every couple laps, until you get some Pamtre Berries. The last two poffins should be made with those, as they provide 19 points of beauty, a small amount of cute, and 34 sheen. After one such poffin, the sheen is at 250, but the key point is that even if the last poffin goes well over the 255 limit (to 284 in this case, rather than the 264 you would get from 11 Wiki), you still get to count 100% of the value of its stats. In fact, you could do even better for the last one by using a Micle Berry (29 beauty, 59 sheen), but that berry was only available through event distributions, which ended years ago for DS games, so it's even less accessible than the Apicot. As a baseline, you'll end up with 9 * 14 + 2 * 19 = 164 points, which isn't quite there, but it's close enough that you only need to gain 6 more points from speed to reach the key number of 170.
Luckily, since you're playing Platinum rather than Diamond or Pearl, you don't actually have to bother with poffin cooking or tracking down berries at all. The developers saw feedback about how terrible the poffins you were limited to as a solo player, and for the third installment they decided to give you a better option, one that would make Milotic unambiguously usable even if you didn't have anyone to link up with. Go to the department store in Veilstone City and head down to the newly finished basement. Here you can buy some berries including the Wiki, but more importantly you can buy pre-cooked poffins with any combination of two flavors that you want, and these poffins are much better than what you could cook by yourself (in fact, they're pretty comparable to what you would get with three players working in cooperation). Just buy 6 poffins with Dry in the name, regardless of what the other flavor is, and that'll be enough. If your Feebas has a nature that boosts Sp.Atk, you can even get away with just using 5.
(If you're playing HeartGold or SoulSilver, there are no poffins in those games, but you can still find Feebas, and they included a couple mechanisms to increase the beauty so you can still evolve it anyway. The haircut brothers in Goldenrod Underground, and Blue's sister in Pallet Town, each offer their services once per day, though in the Pallet case this is restricted to one hour from 3-4 PM. Using those services will increase both beauty and sheen, but unlike single-player poffins, the beauty increase is greater than the sheen increase, and Feebas will always be able to max out its beauty there if that's the only thing affecting its contest stats.)
