Clearly you don't understand the Smogon tier system yet.
The tier system isn't based on evolutionary stages; it is based on viability and usage. Firstly, the Little Cup tier is not the lowest tier - Neverused is. Little Cup is more of a separate tier as it follows many different rules.
As for the Ubers tier, yes, it is the highest and contains the Pokemon that are too powerful for the lower tiers due to how overpowered they are. The tier is 100% legendaries apart from Blaziken and Excadrill, who are there for their own reasons.
Back to the question. There are middle tiers; their names are Overused, Underused, Rarelyused and Neverused. Ubers and the tiers listed just before are all playable metagames; however, there are some unplayable tiers. They are Borderline, Borderline 2 and Borderline 3 (which currently does not exist due to all the Pokemon in it moving to other tiers). They speak for themselves really; Pokemon that are too weak for one tier but too powerful for another are placed in there tiers.
I'll now list all the tiers in order to hopefully explain thing more clearly.
- Ubers. The highest tier, which contain the most broken and OP Pokemon in the game. This tier is made mostly out of Overused's ban list.
- Overused. The second highest tier and also the most popular of all to play due to its balance of powerful-but-not-overwhelming Pokemon.
- Borderline. Pokemon deemed too good for Underused but not particularly good in Overused live here. This, like the other BL tiers, is unplayable. The Pokemon in here are for use in Overused and Ubers.
- Underused, the third main tier. Pokemon not good enough for life in OU are kept in this tier. Generally, UU Pokemon suffer from loss of movepool of bad base stats or typing which hold them back from higher tiers.
- Borderline 2. Exactly the same as BL except the Pokemon are too good for Rarelyused but can't find a place in Underused.
- Rarelyused. A quirky tier with Pokemon better than the worst but still lack a placement in the higher tiers for whatever reason. Probably the least popular tier.
- Borderline 3. This BL tier once existed but now does not as its only member, Jynx, dropped to NU. Same rules as the other BL tiers, except this time with the Pokemon being too weak for RU but too good for NU.
- Neverused. The lowest of the lows. There Pokemon suffer from multiple issues such as being too reliant on something to survive, having bad base stats, or are a gimmick that has little to no usage like Spinda. Gotta love base 60 in every stat. Don't get the wrong idea though; some of the Pokemon in this tier still see usage in higher tiers; in fact, some are even past members of some of the higher tiers.
Little Cup is a separate tier that follows different rules and should not be seen as "the lowest tier".
Also a few last things to note.