"Ich möchte" doesn't really correspond to "I want", but more to something like "I would like...". "I want" would be more something like "ich will", and that would actually be too demanding in a restaurant situation in my opinion.

Using "ich möchte" in a situation like that may be on the more demanding side of things, but is still completely fine in my opinion. Possible alternatives are "ich hätte gerne" ("I would like to have") or just your order followed by "bitte" ("please"):

Ich möchte ein Stück Kuchen.

Ich hätte gerne ein Stück Kuchen.

Ein Stück Kuchen, bitte.

"Ich mag", by the way, isn't commonly used to order something, but more to express preference or fondness.

Ich mag Kuchen. Ich möchte ein Stück Kuchen.

I like cake. I would like a piece of cake.

"Ich möchte" doesn't really correspond to "I want", but more to something like "I would like...". "I want" would be more something like "ich will", and that would actually be too demanding in a restaurant situation in my opinion.

Using "ich möchte" in a situation like that may be on the more demanding side of things, but is still completely fine in my opinion. Possible alternatives are "ich hätte gerne" ("I would like to have"), "ich nehme" ("I'll take") or just your order followed by "bitte" ("please"):

Ich möchte ein Stück Kuchen.

Ich hätte gerne ein Stück Kuchen.

Ich nehme ein Stück Kuchen.

Ein Stück Kuchen, bitte.

If you consider "ich möchte" to be too demanding, you can also "soften" it with a "bitte":

Ich möchte ein Stück Kuchen, bitte.

Same for "ich nehme":

Ich nehme ein Stück Kuchen, bitte.

"Ich mag", by the way, isn't commonly used to order something, but more to express preference or fondness.

Ich mag Kuchen. Ich möchte ein Stück Kuchen.

I like cake. I would like a piece of cake.