HCA: Hermit Crab Association

I have 10 gallon tank with 30-40 gallon uth mat which some part of the mat stick out I also have thermostat to control the temperature of the mat but the mat temperature is 30c when the tank only reaches 27c I also have tin foil on the back of the tank where the heat mat is is the temp suppose to be like that

Yeah, the number of gallons on a UTH refers to the way they are used for reptiles - on the bottom of the tank without any substrate. For crabs, we kind of use them 'off label' by putting them on the back or side. The heat dissipates differently when used like we do, so we recommend going by measurement rather than the number of gallons listed.
What do you mean that 'some part of the mat stick out'?

I have uth mat that's to big for my tank so that big extra bit stick out the top of the tank I had biggest ahsehive one they had

If your mat is one that has an adhesive to stick it onto the tank, you cannot put anything over top of it to insulate because it is a fire hazard. I'm also not sure I would have it so that all of it is not stuck on the tank. Are you somewhere that you are able to purchase one of the ones that is held on with tape? Reptile Basics and Bean Farm have excellent UTH, and they can be insulated over top if needed.