Could I make a virtual pet that evolves based on what users log?

Yes. Start with a few stored values for the pet, such as energy, hydration, and mood, plus the time they were last updated. When someone opens the app or enters data, calculate how much time has passed and apply your rules for that period.

Then use thresholds to decide what the character looks like or whether it changes state. You can cap values and decide what happens after a long absence. Saving the state and timestamp lets the simulation catch up without needing to run constantly.

The hard part is probably making the pet worth caring about, not calculating meters. Give it a few visible reactions and some possible evolutions, then see if the connection between entries and changes feels understandable without being completely predictable.

I'd also think carefully about death from missed updates. Consequences can make the pet feel alive, but a daily obligation can turn logging into a chore. Maybe neglect changes its mood or appearance before anything permanent happens.

Keep the simulation separate from actual health advice. Calories and movement alone can't tell you exactly how a person's body should look, and food quality is more complicated than one number. Make the effects clearly game rules rather than a claim that the character represents the user's body.

For absurd inputs, decide whether they're invalid data or part of the game. Rejecting an impossible entry with a chance to correct it is different from deliberately letting an exaggerated entry trigger a silly outcome. Either is doable, but accidental typos shouldn't have to kill a pet.