Is this grammar correct?
Of the two football players, Herman was the heaviest.
No, it's not correct. You are comparing two people; therefore, you should use "heavier" rather than "heaviest."
In the late 1990s, the popularity of MTV’s The Real World sparked increasingly interest in this concept.
you need to change the word increasingly to increasing. This has to be an adjective to modify "interest", not an adverb.
The correct sentence would be: "Of the two football players, Herman was the heavier."
To determine which word to use, you need to consider the comparative form of the adjective "heavy." When comparing two things, you use the comparative form, which is "heavier." The superlative form, "heaviest," is used when comparing more than two things.
As for the second sentence, it should be: "In the late 1990s, the popularity of MTV's The Real World sparked increasing interest in this concept."
Here, we use the adjective form "increasing" to modify the noun "interest." This indicates that the interest is growing or becoming more prevalent. Using the adverb "increasingly" would modify the verb "sparked," but we want to modify "interest."