DARN IT! I deleted my post because I thought the last post was in December 2018, but that was a register date so... Here I go again...
Here we go again. Read below.
That depends on what you mean by "inferior". If you mean in terms of hunting and fighting ability, that would largely depend on the age of the cat and perhaps their genes, but the latter is an entirely different topic. In regards to the former, I definitely see an older cat wishing to join the Clans having a much more difficult time adapting to "Clan standards" versus if they were just a kit and thus had "unlimited potential". However if you mean in terms of loyalty, then the answer is yes, outsiders are inferior, but only when comparing them to the Clans or any other group for that matter. Knowledge and understanding are two totally separate things and they should never be confused with one another. An outsider might memorize the warrior code (again, using that as an example) but they'll lack the heart and the spirit, so they'll never "get it".
Some fan reinterpretations of the series paints kittypets as objectively inferior in terms of intelligence as well as fighting and hunting ability, while rogues and loners are simply incompatible with group life, no exceptions. While the vanilla series is the only real canon, since Warriors is based off of fiction, I believe that regardless of that fact it's only one of many possible interpretations. Warriors isn't real so for all we know kittypets as a whole might have an IQ of -50, or perhaps some are as capable as any cat, laziness aside.
Are groups like the Sisters, The Tribe, and if you want to count them in as well, WarriorClan, objectively inferior to the Clans? The answer is no. Not a hard no however; you simply cannot compare them against each other. Is an apple inferior to a loaf of bread? Is an elephant inferior to a crocodile? At the end of the day loyalty and caring about others within your own group makes the difference between a rogue gang and a Clan, Tribe, etc. Many cats have developed morality and have practiced honorable deeds without StarClan or the Warrior Code guiding them, something I believe the 5 Clans would have trouble believing.
I still find it an interesting topic to explore however. Most of my OC's are Clanborne and have an inherent hatred towards outsiders, but mostly just to add some extra drama to their biographies.
I personally believe it is shortsighted to have a xenophobic nature towards any group. That doesn't mean distrust isn't allowed however.
I know you're asking in general, but personally if I were Clan leader I wouldn't allow outsiders to join my Clan. The reason being for this is I wouldn't take a chance with an unknown and possibly hostile entity wishing to do harm upon my Clan, but also because like I mentioned above, while they might be able to grasp the warrior code within their heads, their hearts and souls would lack that understanding and thus would just be inferior in that regard. The same could be said for a Clan cat joining The Tribe or vice versa, for example. Unlike IRL (I'm very much pro diversity), I believe that in the Warriors universe each cat should stick to their own little bubbles; it's what they know best.