The first thing in most tub installs instructions is to ensure that it is level...yes, an incorrect slope will likely give you problems both with surround and down the road as water pools rather than runs into the tub. If the subflooring isn't level, you either make it level, or more common, use some cement based product underneath to make it level. Lots of discussions on that topic in the forum, you can search on it.
The tile council of North America (TCNA) has guidelines on how to install tile (generally accepted as the industry bible on how to do things), and the first thing they say is to prep the floor to industry standards prior to laying tile, so there's no excuse for uneven tile. Depending on the size of the tile, the least stringent spec calls for flat within 1/8" in 10', with no variation of more than 1/16" in 2'. If the tile are bigger than 15" on any side, it gets even more strict . There is also a standard for the maximum amount of lippage allowed...you'd need measure what's there to see if it is within specs. Those specs also call for TWO layers of plywood, installed in a particular manner along with the joists being strong enough for stone, which requires at least twice the stiffness of a ceramic. On top of the ply, you generally need either a membrane or something like a cement board (cbu), that also must be installed per industry standards to work properly. Someone taking your money to do a job should provide a result at least as good as the minimum industry standards. That has not been met. There are specific specs in the TCNA guidelines that you can measure the tile job against. You can point to the installation instructions on the tub for the level part. For long-term reliability, how the tile is installed in the surround also needs to be done at least according to one of the accepted procedures in that handbook, as well.
