もうこれ以上はないぐらいに is a partial sentence that needs some sort of adjectival phrase to modify. It means something like "couldn't be Y", where Y is an adjective in comparative form.
Literally, it means, "it is almost like（くらいに） there is nothing（ない） above （以上）this（これ)", so the phrase as a whole acts like a modifier to make a superlative.
これ以上ないくらい幸せ = (I) couldn't be happier! = 最高に幸せ
これ以上ないくらい悪い = (This) couldn't be worse! = 最悪
So to figure out what the full sentence means, we will have to look at the phrase before or after もうこれ以上ないくらいに。