Should I tell the college I'm applying to this?

Well i do not think that your university could forgive bad grades using sickness as an excuse. So why telling the university?

On the other hand i would be more concerned about the fatigue. Your mono should be resolved by now. Unless you are suffering a relapse or you could have another disease. You should seek more medical help, i recommend a second opinion. Ask to peers or family members for a trustable med on your area and go for a check up.

Many diseases can be confused with depression. I could easily give you a list, but you rather look for an objective dyagnosis in person. Again the best solution is a second opinion.

Everybody gets a bit depressed now and again so there's no need to tell anybody..