Yes, after Diploma you may join ISRO after clearing respective shortlisting rounds and interview, subject to vacancies and advertisements for Technician/Technical Assistant Positions.

As far as Scientist position is concerned, after Diploma, even if you pursue a B.Tech/A.M.I.E/Associate Degree in Mechanical/Electronics youll have less chances, as the Vacancies are scarce and competition is extremely high for B.Tech Graduates.

You should perhaps pursue a M.Tech from some of the Higher Technical Institutions like Top IITs, NITs, BITS Pilani or IISc Bangalore followed by applying for the Postgraduate Vacancies, which are more in number and Postgraduates of Premium Institutes are preferred for the Positions of Scientist.

Ironically, at ISRO, the vacancies of Mechanical/Electronics Engineers are 10x more than Aerospace/Aeronautical/Material Engineers, though yhe latter play a key role in the Engineering and Research of ISRO.