‘Am I wrong?’
No. The result is not always as simple as it might be.
In R2021a, the Symbolic Math Toolbox apparently simplifies this result automatically. In situations where that does not occur, call simplify after calculating the result to get a simpllified final result.
syms y(t)
ode = (diff(y,t))==((((t^2)/2)-(y))^(1/2));
cond = y(0) == 0;
ySol(t) = dsolve(ode,cond)
ySol = simplify(ySol, 500) % Equivalent to: simplify(ySol, 'Steps',500)
.
